This short work is intended for anyone interested in stereoscopic pursuits, but particularly for those who are trying to create stereoscopic images. We hope that the words and phrases to be found here will become useful tools rather than obstacles in your approach to "3-D" imaging.

We stress that this is a compendium of terms that we have found useful, confusing and/or interesting; while we have tried to make it accurate, we do not represent that it is complete or of great scholarly depth. Some entries are long, because we found the subject interesting or we knew a lot about it; some are short for the obvious other reasons! We have tried to keep the tone conversational, rather than strive for uniform and rigid style; we've not felt ourselves above an occasional wise-crack. Additions, corrections and improvements are always welcome --- we see ourselves as editors rather than authors.  (Glossary is available via diskette or e-mail.)
 
 

* ANSI: American National Standards Institute.  See the next entry.

* ASA: (1) American Standards Association. Although the expression "ASA" is still applied to U.S. film speeds, the "American Standards Association" changed its name to the "American National Standards Institute" in 1969. Their standards are referenced by ANSI numbers such as "PH3.11-1953" (which describes the 5p format used in cameras like the Stereo Realist and such as the Kodak Stereo 35 cameras).

(2) As a film speed, it now appears in conjunction with the European DIN number (see) in the format "100/21" which, as such, is the "ISO" speed. Relative film speed is proportional to the ASA value.  Absolute film speed is determined by a formula which in turn describes the judgment of a panel of experts who have been shown the results of carefully controlled trial exposures.  If you don't like the results of using the prescribed speed, use another and term it your "EI" or "exposure index.  Photographers often prefer to use higher EIs for transparency films (to prevent color wash-out) and lower EIs for negative films (to ensure adequate negative density).

* Accessory lens(es): Lenses that --as with planar cameras-- enable the stereo camera to focus and/or converge closer (see: "Angle lenses").  Close-up lenses allow close focusing without the complications of excessive lens extension, need for exposure compensation, and the like.  Other accessory lenses include those which alter the angle of acceptance, producing in effect modest wide-angle or telephoto effects.

* Accidental Stereo Effects: Those stereo effects encountered by --for example-- fusing two postage stamps with printing irregularities or encountering a pair of photographs in which there has been incidental lateral displacement between the two exposures.  Sometimes called a "found stereo" by analogy to that venerable genre, the "found poem."

* Accommodation: The refocusing of the eyes as their vision shifts from one distance to another.  When using a stereoscope or "free-viewing", accommodation is uncoupled from "convergence"; those two processes are normally linked to one another as a reflex.

* Achromatic (lens): Lenses designed to avoid chromatic aberration.  Simple one-element lenses and prisms, when focusing rays of "white" light can't bring the rays to a single point (or image made up of points).  Instead, a family of images (a blur) consisting of differently colored versions of the image/point(s) are created.

An achromat brings the main colors contained in white light to a common sharp focus.  This is achieved by having two or more lens elements, which differ in shape and/or refractive index, such that their chromatic aberrations tend to cancel each other out.  For example, a mildly diverging (minifying; minus-diopter) lens of high refractive index may be placed in front of a stronger, converging lens of lower refractive index.  The red end of the spectrum will be diverged more (than the blue end) by the first element, but also converged more by the second one --- if the balance has been well chosen, all colors will focus at the same place.

* Actual image: An optical image which is "actual" in the sense that it can be shown to exist by putting a screen at its position and seeing it in projection.  It is sometimes also called a "real" image.  The image in the camera at the film plane is an example.  (As distinguished from a virtual (see) or "Space" image.)

* Acuity: See: "Stereo acuity".

* Aluminum Screen: Usually also with a "lenticular" surface (diamond pleated so as to disperse light back into the audience), the aluminum coating preserves the polarization of the two beams from a stereo projector.  A white mat or glass beaded screen won't.  For rear projection, simple ground glass or special lenticular glass screens (ie: actually micro-lensed, and for dispersion) can also preserve polarization.

* American stereoscope: See: "Holmes".

* Anaglyph: A stereogram in which the left and right images are superimposed but printed in complementary colors (often red & blue-green).  It is decoded and viewed by placing correspondingly colored filters over the eyes. The most practical form of stereo illustration for the printed page.

* Analyzer: See: "Decoder".

* Angle lenses: Supplementary lenses for a stereo camera used for close-ups. They both focus and converge the camera's design window to a closer plane. A single large lens covering a pair of camera lenses will serve this purpose.

* Aperture: Any optical opening such as a camera iris or that at the back of a camera that masks the film and determines the frame format.

* Auto-stereogram (graph): A stereo image that requires no auxiliary device or technique to be viewed (e.g.: Nimslo lenticular prints).

* Axis: See: "Photographic axis," "Optical axis," "Visual axis," "World War Two, Major Alliances of.

* Bands: Ghost zones seen at the sides of an incorrectly trimmed/masked view.

*  Base: Usually understood to be the distance between the left and right lenses when a scene is stereographed.  In this case, "stereo base" is roughly equivalent to the term "stereo displacement, although stereo displacement is sometimes expressed as an angle (see illustration), especially with medical X-ray stereo pairs.

It's the author's opinion that stereoradiograms, made by grossly rotating the patient between exposures are normally terrible. Were they made by means of laterally shifting the plates and/or the X-ray head, less frequent resort would have to be made to the expensive apparatus and their dedicated rooms/staff for advanced techniques such as magnetic resonance imaging (MRI).

Sometimes the subject gets slightly rotated on a turntable or rocker (hopefully, 3 degrees or less) between exposures, which are made with a single lensed camera (a technique often used in stereomicrography). If a small angle of rotation is used, the results bear some resemblance to that of a camera base of "b" divided by "d" (above illustration) --which gives the same angle in radian measure, or b/d times 57.29 --to get that same angle (of rotation) in degrees. More formally: angle of rotation = 57.29 (arc-rad [b/d])  --up to 3 degrees (or so).

A number of 3D movies have been made using cine cameras in which two lenses are adjustably "toed in" (for the best stereo window), as well as being displaced left-to-right. Both toe-in and rotation introduce registration problems and depth distortion in some proportion to the angle/s used.  See discussion under "Keystoning".

* Bates, Joseph: The man normally credited with producing the first of the classic American ("parlor") stereoscopes, complete with a hood and an adjustable stage/card holder.

* Beam splitter: (1) An optical element that reflects and passes specified percentages of the light rays striking it. For example, a beam-splitting prism is often employed in a microscope to allow viewing of the image through the eyepiece(s) at the same time one is photographing it through a separate upright tube.  The old Canon "Pellix" camera used a pellicle-mirror-type beam-splitter instead of a moving mirror, to allow SLR function without mirror "slap"; this technology has recently been improved, and reintroduced in the Canon EOS-RT.

(2) A "stereo attachment" (see); technically, most prismatic or mirror stereo attachments are more properly termed image splitters or frame splitters, as they do not split an individual beam into components. The difference in light path between
the two devices is illustrated below.
 

* Binocular instrument/viewer: Any 2-lensed viewer, stereoscope, or a device for viewing a planar image with both eyes.

* Binocular vision: --implies, but might not refer to stereoscopic vision --which requires the healthy function of a discrete area of the brain which sorts out relative parallax differences between the eyes.  See the "Cyclopean", "Stereopsis", and the preceding entries.
 

* Brewster, Sir David: A great man of many accomplishments among which was the invention of the kaleidoscope (1816) and the lensed stereoscope (presented in 1849, and published in 1856 in his The Stereoscope).  This he referred to as the "lenticular" stereoscope as opposed to the earlier Wheatstone mirrored (but unlensed) stereoscope (which see).  He carefully described the use and collimating nature of off-axis or prismatic lenses, particularly the use of lens segments ("semilenticular" elements) for such purposes.  Consequently, all lensed 'scopes can be referred to as "Brewster stereoscopes", whether full or semilenticular, but a fully lensed stereoscope (whether or not used off-axis) is frequently referred to as a "Claudet" 'scope.

* Camera axis: See: "Photographic axis".

* Cancellation: The mutual extinction of the unwanted images (i.e.: no vestigial "left" image should be seen by the right eye) in a stereo viewing system where the presented image is a coded composite of both the left and right components.

* Carbutt, John: The first production 35mm camera (the "Homeos") was a stereo camera and stereoscopy preceded photography itself; thus, it is only fitting to note that the innovation and first use of dry plates and celluloid film was in the field work of the pioneering stereographer John Carbutt --not, as commonly believed, by George Eastman and the Kodak labs.  (See: John Carbutt in On the Frontiers of Photography by Brey.)

* Cardboarding: The characters in hand-drawn stereo cartoons look as though they are cut out of cardboard. Sometimes views stereographed from life also convey this impression, especially if there is "squeeze" distortion and/or if there are few distance cues intervening between major subjects.  This impression of discontinuous depth is called "cardboarding" in stereographer's jargon.

* Chip: (jargon) One of a separated pair of transparencies (usual) or prints (occasional). The term probably derives from the small slide frames of ViewMasterTM reels. Sometimes this is also called a "frame, although it causes confusion with other uses of that word.

* Chromatic aberration: The degree to which light is bent when it passes at an angle through a transparent material such as glass is not the same for all colors. A simple lens will therefore not focus white light to true lines and points, but rather to a family of colored images blurred together.  Different colors will find their sharpest focus at different distances from the lens.  When a lens is used off-axis --as is the case with the common print stereoscope-- this effect may be particularly prominent, and notices as a colored fringe around a seemingly sharp image. However, the degree of prism effect in such lenses (= 6 diopters) is such that the effect is usually not objectionable. Compound lenses may use elements of different refractive index to offset this aberration (See entry under "Achromatic lens").

* Chromatic stereoscopy: An effect resulting from color aberrations in viewing lenses (or even in the eyes) which produced stereoscopic effects in both stereo and planar color images.

* Circle of Confusion: (1) The smallest image that a camera's lens can form of a point of detail.

(2) The smallest such image of a point of detail that is assumed to be acceptable for the calculation of depth of field, depth of focus,
"hyperfocal "distance and related indices. This is often taken as 1/1000 inch, and standard depth-of-field tables are based on that assumption. A smaller circle of confusion assumption (which may be appropriate for very high resolution lenses and films) will yield shallower depth-of-field estimates than those provided by the conventional tables; a larger circle of confusion (which may be appropriate for soft-focus portraiture) will yield deeper depth-of-field estimates.

* Claudet, Antoine: was a Frenchman, but privileged to be Queen Victoria's court photographer and a  contemporary of Brewster and Wheatstone.  He operated a stereoscopic portrait studio and produced (4) viewers with complete (i.e. not "semi"lenticular) viewing lenses.  He was probably the first to recognize the distortion caused by the use of long-focus camera lenses "compensated" with a wide base.  This causes a combination of two distortions, "Squeeze" (contracted perspective) and the special distortion of stereo minification. This combined distortion is now often called by the jargon term "PePax", a contraction of perspective and parallax coined by Herbert McKay (see) many years later.  This is also sometimes called "frustum effect.

* Co: Gender-nonspecific pronoun: co (nominative), cos (genitive), com (dative/accusative)

* Colardeau Progression: In a standard 5-perforation format camera (Realist, Kodak Stereo 35), the lenses are separated by three frame-widths, and the film is advanced by a constant two frames.  This results in stereo pairs separated by two unrelated frames on the film (a clever idea). This progression sequence was designed by L.J.E. Colardeau, who with J. Richard produced in 1914 the Homeos stereo camera using it.

The advantage of the Colardeau progression is that the film travels the same distance each time it is advanced, allowing a much simpler and more reliable film advance mechanism.  The minor disadvantage is that the individual frame width is limited to one third of the distance between the centers of the left and right images of a given exposure. For example, the Realist had its two images on 70mm centers; thus, the Colardeau progression required that the frames be no more than 23.33 mm wide (so if you ever wondered how the Realist, Kodak Stereo and several others all ended up with 23x24 mm frames, that's the "scoop").

* Collimation: When the distance between the centers of a stereoscope's ocular lenses and the separation of the image pair placed in it are equal, and when the images (or view) are (is) placed at the full focal length, the lenses of the stereoscope act agreeably as "collimators" for the "homologous" points of the image pair.  This means that the interpupillary distance of any user's eyes is automatically accommodated without adjustment.  It also means that one's eyes need not be perfectly level with the view (a great convenience in viewing). (Accordingly, gravity-leveled eyewear has been proposed for viewing side-by-side pairs presented on a fixed screen at close range.)

* Convergence: The meeting of lines of sight through the eyes or of light rays through an optical system at a common target point.  Sometimes this is expressed as absolute convergence --the angle subtended by the converging lines at the target-- but also as half that angle (the convergence for each eye rather than for both) and as relative convergence.  See: "Deviation".

* Convergence micropsia: Minification (image appearing smaller than it ought) resulting from the exaggerated parallax of hyperstereo.  This is sometimes called "miniaturization" in stereographers' jargon; that term, however, is more properly applied to making an object physically smaller, rather than making an object appear smaller.  See: "SILO Effect".

* Copilia quadrata: Worth a mention in anyone's glossary, the female version of this pinhead-sized aquatic creature was first described in 1891 by Seliig Exner.  Between 1963 and 1972 its bizarre optical equipment (and mating behavior) was recognized and examined by Richard Gregory (see his Odd Perceptions).  Copilia is quite transparent and the entire width plus more than half her length is devoted to the support of a binocular visual system! Even more striking is her animal kingdom exclusive: she has interior single element photosensitive arms that linearly scan the image pair and "flyback" (Television style) to rescan another line of the image pair!!

* Corresponding points: See: "Homologous points".

* CraigLo Camera: A minor modification of the Nimslo camera consisting of narrow inserts, glued to either side of the center focal plane septum (ie: between frames #2 and #3) --such that the operator of a manual film printer is encouraged to jog the film --enough to neatly and properly print the outside pair of the quad --plus enough of the inside pair to fill the printer's 8 perforation (35mm "full frame") aperture mask. (This modification is probably of no avail for today's (2005) fully automated processing machines. Consider using recent editions of the "Loreo" or "Snap 3D" stereo cameras.)

* Crossed-eye viewing: See: "Free-viewing".

* Curved cards: Curved cards became popular late in the last century. Such cards are stronger, protect the image pair, and "preëmpt" random warps about either axis of the view card.  It has been further suggested (in an interview of a Keystone engineer by Prof. Max Kent, of Dayton, Ohio) that the curve helped match the view's surface to the field curvature of the stereoscope's lenses.  In Stereo Views, author John Waldsmith reports that curved cards were first sold to the public as a gimmick, claiming that they had a superior depth content.

* Cyclopean: (From Cyclops, after the [Greek] mythical one-eyed giants) (1) Pertaining to a cyclops.

(2) Metaphorically, pertaining to the fused stereoscopic image, as opposed to a non-stereoscopic image or to the stereo image as its two or more planar component parts.  Thus the:

* Cyclopean axis: The apparent axis of view of a stereoscopic image; generally, the line bisecting and coplanar with the angle made by the two converging optical axes of the eyes (or the line parallel to and midway between the two optical axes of the components of a strictly orthostereoscopic system such as a traditional stereo camera).

* Cyclopean center: (1) The area of the brain which perceives a fused stereo pair as a single, three-dimensional image....the "mind's eye," if you will.

(2) A social hall for one-eyed people, of little interest to stereographers.

* Darrah, William C.: Author and publisher of the definitive The World Of Stereographs in which views and makers are compiled with a worldwide historical scope.

* Decoder: See: "Viewer.

* Depth of field: Although a camera lens truly focuses at only a specific distance, objects closer or farther than that specific distance may be in very acceptable focus.  This range of acceptable focus is known as depth of field. Once the acceptable criteria for sharpness have been set (generally as a maximum "circle of confusion [q.v.]"), the depth of field is determined by the lens aperture ("f-stop") and by the ratio of image-to-object size; the latter is determined in turn by lens focal length and the distance from the camera to the object.  To maximize depth of field, one would use a small lens aperture (large "f-stop" number), a short focal-length lens, and a long distance from camera to subject.  In stereography, one of the seldom intentionally-broken "rules" is that of keeping everything in the view in sharp focus.  One expects to be able to focus at different depths in a stereo image, and it can be very distracting if it's not possible --the use of shallow depth of field for effect in stereo rather often backfires.  Comment: It is commonly held that a short focal length lens gives greater depth of field; that's really a half-truth.  If one takes a full-frame head-and-shoulders portrait with a 105 mm portrait lens at f:11, then takes another --also full-frame head-and-shoulders, same image size --with a 35 mm lens at f:ll, the depth of field will be the same in both photographs (though the nose will not be flattered in the 35 mm view).  The 35 mm lens will give greater depth of field if the distance to the subject is what's held constant, rather than the image size; in that case, one pays for the increased depth of field with a smaller image size.

* Depth of focus: This is the same phenomenon as depth of field (see), but at the other end of the optical axis.  In other words, it's the acceptable error range for film positioning. If a lens does not bring the image to focus in a true plane at the film-plane of the camera (as when using a non-macro-corrected lens for macro work), edge sharpness will be enhanced by the use of a smaller lens aperture to maximize depth of focus.

* Depth perception: See "Stereopsis".

* Deviation (sometimes "deviation range"): In stereoscopy: the range of convergence within a view (see also "MiRF" and "Mer"), but only the angle turned by one eye between near and far points.  Ophthalmically: deviation (and convergence) is measured in diopters (which see). 2.5d (=1.43 degrees or the deviation for an apparent range of depth from 4.3 feet to infinity) has been suggested as a limit for good stereographs.

* DIN: Deutsche Industrie-Norm A system of standards, analogous to that of the ANSI (see).  For film speed, the DIN numbers are different from the U.S./ANSI/ISO/ASA numbers in that the scale is logarithmic rather than linear. Thus, for each doubling of film speed, the DIN rating increases by three degrees.  ISO/ASA 100 = DIN 210; ISO/ASA 50 = 180; ISO/ASA 200 = DIN 240

* Diopter: (1) The power of a lens found by dividing its focal length into 1 meter (very useful for dealing with optical systems).

(2) The power of a prism (or the off-axis prism equivalent of a simple lens) found by tracing the deflection of a ray passing through it.  The actual deflection (say: to the right on a screen beyond the lens) as a percentage of the distance to the screen is the prism power in diopters (=100 times the tangent of the angle of defection).

(3) A measure of the convergence or deviation of the eyes.

* Diplopia: Double vision.  Normal to all images visually perceived as being closer or further than the convergence point of the eyes.  Abnormal in vision when image fusion (and stereopsis) can't be achieved.

* Disparate images: A pair of images that fail as a stereogram due to distortion, poor trimming/masking, mismatched camera lenses, or the like.

* Divergence: The opposite of convergence, as when the eyes' lines of sight "toe out" from each other.

* Dynamic stereo diplopia: The active convergence and divergence of the eyes along planes of depth during stereoscopic vision.  If a stereograph or a scene in life is viewed for only an instant, or if the eyes be stilled in their orbits, there is no stereo vision.  Like the other senses, the experience of stereopsis depends on change and refreshment.  (This is but one side of an old debate that started between Brewster and Wheatstone and carries on to this day.)

* Eclipse system: A stereo viewing system in which the left and right images are viewed alternately in rapid sequence.  Persistence of vision causes the images to be blended in the mind's eye.  A viewing device decodes or separates the images by alternately interrupting the vision in each eye in synchrony with the screen sequence.  This is the probable future of 3-D TV; current devices use electroöptical shutter spectacles at a field rate of 120 per second (= 60 interlaced frames).  The technique has also been used in cinematic and still projection by using mechanical shutters.

* Elmo (attachment, system): An over/under image splitter stereo attachment. As the name implies, this attachment was made to fit Elmo brand 8mm motion picture cameras and projectors.  It is similar to the Savoye device in that it places stereo pairs onto a single frame in bottom-to-bottom relationship, but uses only front-surface mirrors to accomplish this.  Elmo --of Japan-- never officially exported this device to the U.S., reportedly due to a concern that it might damage the reputation of Japanese manufacturing (i.e. Americans were thought not to have the patience and discipline to adjust and use the system properly).

* Emde: a brand of die-cut aluminum slide mounting mask; the product line includes normal (RealistTM-format) stereo masks, masks for European format stereo cameras and masks with smaller apertures for the correction of stereo window in close-up views.

* Esophoria: A tendency of the eyes to converge at rest.  This is not as common a condition among normal people as is a slight degree of exophoria (see). This is advantageous in free-viewing stereo pairs in crossed fusion (reversed left-for-right), but disadvantageous in viewing orthotopically-positioned stereo pairs.  Esophoria and exophoria may be unmasked by covering one eye and fixing on an object; when the covered eye is uncovered, it will be noted to have drifted and will have to correct its position to allow proper fusion; the subject will notice transient double vision.

* Esotropia: An abnormal tendency of the eyes to converge.  This is basically the extreme case of esophoria (see); the difference is that no cover-uncover or cross-cover test is needed to demonstrate it, and it may make it impossible to achieve stereo fusion.  It may also lead to the underdevelopment of vision in the non-dominant eye, a condition known as ambylopia ex anopsia (and sometimes called "lazy eye" in common parlance).

* Exophoria: A tendency of the eyes to diverge at rest.  Many --- perhaps most --- people have a slight degree of exophoria, such that a very small amount of convergence from the rest position is necessary to view objects at infinity.  This is advantageous in free-viewing orthotopically-positioned stereo pairs (not reversed left-for-right). People who have a bit more exophoria than others tend to find such free-viewing easier, and to be able to free-view images at a larger stereo separation.  This is a disadvantage, however, for the free-viewing of stereographs in the cross-eyed mode.  Esophoria and exophoria may be unmasked by covering one eye and fixing on an object; when the covered eye is uncovered, it will be noted to have drifted and will have to correct its position to allow proper fusion; the subject will notice transient double vision.

* Exotropia: An abnormal tendency of the eyes to diverge.  This is basically the extreme case of exophoria (see), the difference is that no cover-uncover or cross-cover test is needed to demonstrate it, and it may make it impossible to achieve stereo fusion. It may also lead to the underdevelopment of vision in the non-dominant eye, a condition known as ambylopia ex anopsia (commonly called "lazy eye").

* Extended legend: Information provided on the reverse side of a stereocard, which extends the simple title-and-maker description characteristically provided on the face of the card.

*  Ferwerda, Jacobus G.: Author of The World of 3-D, (Second edition: 1987).

*  Five-perforation format: See: "Format."

*  Flicker projection: See: "Eclipse system"; not to be confused with the vigorous tossing of a small bird.

*  Floating edges: Top, bottom or side edges of a view which do not agree in location between the two images of a stereo pair.  Such edges are said to "float," because of the sense they give in fused viewing that their position is not fixed, or that they are in the wrong place with respect to the subject matter.  Often the "floating" appearance results from having a "stereo window" which is not all in the same plane, or which is located behind subject matter which is in turn close to the edge of the view.  See the discussion under "Stereo window".

* Format: (1) The dimensions of an image, such as those on a strip of film; for example, the 24x36mm full-frame standard format for 35mm planar cameras.

(2) Reference to format by associated features.  For example, the 23x24mm image size may also be termed the "5p" format, because there are five standard 35mm film edge perforations (sprocket holes) per frame.

(3) In stereography, "format" can also be a reference to the separation of "homologous" details at infinity in a type of view (i.e.: "78," "80," or "82"mm for print views); the overall dimensions of the unmounted view (i.e.: European "6 x 13" centimeter); mounted view ("7 inch" for American print views); or separately mounted image pairs ("[2x2]x2" or "2x2" for paired standard mount "full frame" [35mm 8 perforation] transparencies).

*  Frame: An individual image on a strip of film.

*  Frame notch: See:"Witness notch".

*  Free vision (or free viewing): Usually understood to mean the fusion of adjacent left and right image pairs by the ability to either diverge or hyper converge the eyes (while they remain focused at reading distance) in order that the side-by-side images be seen in register as a single fused view (ie: no use of a viewing device).  This has been seriously proposed as a skill to be learned by the public at large for the purpose of viewing stereoscopic image pairs in mass media publications (see "Herbert McKay").

* Frustum effect: Distortion creating images shaped like a frustum (the term may apply to any solid which has been cut by two parallel planes, but is most commonly used to describe a truncated cone).  See: "Keystoning", "Claudet".

* Fusion: The visual registration in the mind ("Cyclopean center") of the images perceived in each eye.

* Ghost image: (1) A transparent image.  Thus: an image that can't be satisfactorily fused or directly seen, such as the "bands" that appear at the sides of an incorrectly trimmed/masked view.

(2) The "alternate" images that are insufficiently canceled by (say) a decoding viewer of a composite view.

* Grapho(stereo)scope: The "Graphoscope" has no stereo lens pair; only a large single element for examining flat prints.  See "Stereographoscope" for the item you're seeking.

* Grid (stereogram): See: "Parallax stereogram"

* Hammerschmidt Jig: (1) A dance so lascivious that it has been banned in nearly every civilized country.

(2) A turntable stage for stereography of small objects, by which two sequential planar images are made with rotational displacement to create a stereograph. The jig rotates the light sources through the same angular displacement as the subject, so that the shadows remain the same in both images.  See: "Rotational displacement stereography.

* Heterotopic: Not in the customary place or orientation. The opposite of "orthotopic" (see).  An image designed for cross-eyed fusion or reversing stereoscope viewing is a heterotopically-positioned view, while one designed for a conventional Holmes 'scope is an orthotopically-positioned one.

* Holmes, Oliver Wendell (Sr.): A physician and author who, in 1859, is said to have reduced the then expensive and box-like Brewster stereoscope to its bare elements and attached them to an open skeleton frame (see: "Bates"). (Oddly: others say he added only the hood and sliding stage, but his surviving hand made model shows neither.) Holmes was a frequent contributor of essays to The Atlantic Monthly, many of which celebrated stereography.  His son became (U.S. Supreme Court) Justice Oliver Wendell Holmes (Jr.).

* Holmes Library: The National Stereoscopic Association, its members, and Eastern College at St. Davids, Pennsylvania support a fine and growing library and museum of stereographica for research. (In 2006, without announcement, consulting the NSA membership or past donors, the board of the OWH Library sold off the collection --!)

* Holmes-Bates Stereoscope: This is the classic, "standard", "American", skeletal, affordable, mass-produced "parlor 'scope". See also "Lenticular stereoscope".

Although the Red Wing Viewer (which see) is the only standard stereoscope known to the Glossary's authors to have published specifications, most standard 'scopes were and are similar in their designs and geometries --allowing for the fact that people's faces and eyeglasses are significantly larger now than they were 80 to 150 years ago. This is because a standard 'scope's lenses were traditionally made by bisecting a single 60 to 70+ millimeter diameter plano-convex lens, squaring them, then turning and mounting them into the 'scope with the thick ("base") edges outward. Necessarily, the outer edges have to be far enough apart (about 3-1/2" or 90mm, reduced a bit by the lens cell retainers) to accommodate the lines of sight from the pupils of one's eyes as they orbit from left to right --in order to see all of a (up to 3" wide each) stereo print pair on a 7 inch wide card. Old scopes might have a lens span of only 3-3/8 inches or 85mm.

That span between outer lens edges (which might be hidden as mounted) is the "optical centers" (span) of a stereoscope. With a card placed at the full focal length of its lenses, the span between optical centers is the limit of the distance between "homologous points at infinity" (which see) of the viewcard (as trimmed and mounted). In practice, however, people like to "crowd in" on a viewcard when the stage of a stereoscope is adjustable. They haul the viewcard in closer to the lenses in order to "see more", to make the view bigger, and often to accommodate slightly myopic vision.

However, when a card is viewed at less than the focal length, the "prism power" of its base-out lenses --that is: the lenses' ability to diverge the user's lines of binocular sight, is proportionately diminished. Consequently, best practice is to trim and mount view cards to no more than about 3-1/4" (82-83mm) of maximum separation (see: "Homologous points") --for use in a standard stereoscope intended for 7 inch wide cards.

While much is to made about separations, the most critical specifications for viewers and viewcards are the tolerances to which the lenses, stage, and cards are held with respect to the vertical registration of the images. Best practice is to keep the apparent discrepancies in a fine binocular instrument to within 1/4 minute of arc (15 seconds, or about 1/100mm at 200mm to the view card!), but when working with the wood, paper, and mat board of print-pair stereography, you're doing very well to bolt it all down to within half a millimeter over-all. We learn to use/tolerate a little "aphoria" (which see) and almost unconsciously correct to best eye comfort by torqueing (as necessary) on the 'scope, viewcard and stage while viewing. (Note: our eyes are normally biased with a bit of aphoria --which means one eye looks a tad higher than the other. That way the muscles of our eyes work under a wee bit of tension to correct it to true, with no free-floating "backlash".)

* Holography: A technique for producing images which convey a sense of depth, but are not stereograms in the usual sense of providing fixed binocular parallax information.  This is accomplished by the recreation of (spectrally restricted) wave fronts of light that appear as if they emanated from the subjects that were holographed.  It is to date impractical for all but lab-produced examples of this process, since the coherent light source, subjects, and glass-plate-supported microfine-grain emulsion must be held extremely steady in their spatial relationships, free of vibrations or out-of-system illumination.  Somewhat greater flexibility is possible with the use of pulsed high-energy lasers, but cost and safety considerations have limited application of this method. Despite these limitations, and the inability to present accurate color, the images may sometimes be quite striking. See discussion under "Space Image.

* Homologous distance: The distance between two "homologous points" (meaning the same --except for being the left and right versions) in a stereo view.  The maximum separation of such points in a view's distant subject matter are often said to be "the separation" of the view, and held to some standard.  The optical centers of the lenses of a stereoscope (or free hand lorgnette) are separated by this distance or more.

* Homologous points: See: "Homologous distance".

* Hyperfocal distance: For a given lens used at a given "f-stop" with the expectation that it will resolve point details on the film no larger (ie: fuzzier/blurred) than a "circle of confusion" of a given diameter: --a focusing distance can be determined for which all subjects from infinity to 1/2 the focus setting will be "in focus."  In other words (and numbers): take the square of the focal length (in inches), divide that by the product of the "f-stop number" (that you plan to use) *times the diameter of the smallest detail you want your lens to be able to resolve on the film (in inches). The answer (in inches) is the hyperfocal distance.  It's easier to use the DOF indications on your focusing scale, but the resolution figure has then been chosen for you (usually .001 inch).

* Hyperstereo: The effect of enhanced depth and reduced scale of a scene introduced through use of a camera lens (pair) separation ("lens base") wider than normal (65 to 70 mm).  (See entries under "Stretch" and "Squeeze" for effects caused by disparities between taking and viewing distances).  Very large camera lens separations are used to reveal depth in architectural and geological-scale scenes.  The perception of a hyperstereo image is described by the terms "SILO effect," "convergence micropsia" and "Lilliputianism" (all q.v.). "Miniaturization" more properly describes the actual physical reduction of an object, rather than a mere reduction in apparent size, but this term is also often applied.  "Puppet theater effect" (see) is caused by resorting to a perceived image pair infinity that is significantly less than visual infinity.  Studies by David Burder suggest that the static nature of still stereography does not completely serve one's visual expectations if only the "orthostereo" (see) conditions of stereography (1-to-1 relationship between lens and eye separation; 1-to-1 relationship between viewing and taking distances) are employed.  This is because when we look at a scene in life, our heads and bodies are in motion --perhaps even involuntary motion-- as we attempt to explore the parallax changes in a scene caused by changing our point of view. Mr. Burder's tests concluded that modest hyper (and stretch) enhancements to a view that are in proportion to its scale were generally appreciated and considered "normal" by general audiences, and they have often been used by producers of  commercial stereo views.

* Hyponar^TM: A brand of parallel-axis stereo lens set which replaces the lens of an ExaktaTM single-lens reflex camera.  This produces a stereo pair which is reversed left-for-right (because the images are in orthotopic position but inverted), so that a transposing stereoscope is needed to view them without remounting.  A similar device was also produced under the brand name of KinDar^TM

* Hypostereo: Stereography with a camera lens base (lens pair separation) narrower than normal (65-70 mm) is often used for adult portraiture and close-up to macro/micro distance stereography.  A similar  effect (called "Squeeze") can be achieved by enlarging the images and viewing a smaller portion with standard or shorter-than-normal 'scope lenses.

* Image splitter: A device mounted on the lens of an ordinary planar camera, which through the use of mirrors or prisms, divides the image and frame into two halves, which are the two images of a stereoscopic pair.  Sometimes called a frame splitter, and sometimes imprecisely called a beam splitter (see). Such devices have appeared under the Zeiss, Stitz, Franka and Pentax trade names, the latter two in production and fairly readily available as of 1990.

The devices usually produce a small degree of keystoning, which may or may not be noticeable in an individual view. Their advantages are modest cost, portability, the ability to use ordinary film processing services, and the ability without much fuss to mix planar and stereo views on a single roll of film.  The disadvantages are that viewers (O-Stereoscopes [q.v.l) for the resultant (18x24mm)x2 slides are scarce, and the slides can be projected only with similarly uncommon polarized (convergible) splitters (or by splitting them into their component half-frame transparencies); routine 3R or 4R prints into this format may be made into stereo cards, but either require a good bit of trimming (4R) or the individual frames are too narrow (3R) to use pre-cut masks, such as Q-Vu masks.  Because of the modest price, and the ease with which an occasional stereogram may be made, these devices may be excellent "starter" equipment for someone exploring whether he or she really wants to take up stereography.

This image shows a schematic of such a device, as well as a line outline of the appearance of a transparency made with one.

* International Stereoscopic Union (ISU): This is an international organization; its journal Stereoscopy is printed in English.  Some of this glossary has its origins in the (old) "Technical Supplement" to Stereoscopy.

* Interocular distance: The separation between optical centers of a viewer (which may be adjustable).  (From L. oculis, eye; an ocular lens is one through which the eye views, as opposed to an objective lens.  If adjustable, the interocular distance of a viewer's eye lenses is often set such that the lenses' geometric center is equal to the distance between the optical axes of the eyes themselves, whereas they should be set to equal or exceed the infinity seperation of the mounted view slide or card.

In stereography, the term is often used to refer to the distance between the axes of the ocular lenses, which may not be quite the same as the distance between the axes of the eyes in a slide pair viewer, or grossly different in the case of a standard print pair stereoscope (for reasons of collimation [q.v.]).

{Again: it is a common error to assume that the interocular distance should agree with the interpupillary distance of the observer.  It should be equal to or be slightly greater than the maximum separation of homologous points of the view (in the case of a simple viewer) or such points of the image formed by the objective lenses.  See "collimation."}

* Interpupillary distance (IPD): The distance between the pupils of the eyes when vision is at infinity.  IPDs range from 55 to 70 millimeters, but the average is usually taken to be somewhere between 63.5 and 65mm (2-1/2 inches).

* Inverse stereoscopic: The preferred (Soviet [of course, given that a "soviet" is a type of governmental deliberative assembly, one might question the political overtones of using the term in an adjectival sense to describe something which merely derives from the Soviet Union]) term for what is sometimes called "pseudostereo" (which see).  This is the sometimes confusing and at other times intriguing visual effect when the planes of depth in a stereograph are seen in reverse order.  Things far seem near despite their occultation by things in the view that were actually near (i.e.: what you "see" is impossible!).  An instrument called a "pseudoscope" was invented (and claimed by Sir David Brewster [away from Sir Charles Wheatstone] as of 1849 in his 1856 The Stereoscope).  This device produced the same effect in the observer's surroundings.  A simple pseudoscope may be made from two 90" prisms (held as "outside" or "Dove" prisms).

* ISO: See: "ASA"

* Ives, Frederic E.: See"Kromscop".

* Judge, A. W.: Author of Stereoscopic Photography in several editions.

* Julesz, Bela: While working at the Bell Telephone Labs with a background as a RADAR engineer (and a then unseen future as a visual psychologist), Julesz created the first pairs of random element stereo pairs. Little or nothing can be perceived in either frame of the pair but, when visually fused, stereopsis --and stereopsis alone-- slowly reveals that the otherwise randomly placed elements are organized into homologous pairs that form patterns in perceived depth.  This invention/concept and all its variations has ignited a formidable amount of research into visual perception (lately indicating that the brain has a small area dedicated to stereopsis/parallax processing/analysis).  It will not fail to ignite your own wonder at the processing of information that goes on in the mind, quite without our volition or knowing direction!

See Julesz's Foundations of Cyclopean Perception, R.L. Gregory's Eye and Brain, and John P. Frisby's Seeing. NSA member Gerald Marks produces very effective random dot anaglyphs using silk screen techniques.

* Keystoning: When the plane of a projected image falls on a screen that is not parallel with the transparency being projected, the image "spills out" or widens at its further extents in the familiar "keystone effect".  The trapezoidal screen image resembles the keystone of a masonry arch, calling to nostalgic memory the herald of the Pennsylvania Railroad.  When the optical axes (plural of axis) of a stereoscopic projector are misaligned, the mutual keystoning of the image pair can be very disturbing.  Similarly, when the plane of the film in a camera is out of plumb with a building being photographed, the familiar "perspective distortion" of should-be-parallel-but-they're-convergent building lines occurs in the finished picture. If two cameras are synchronized to produce a stereographic pair, AND if they are "toed-in" to converge on a subject (at a finite distance from the cameras), the resulting pair will exhibit mutual keystoning, and consequent depth and shape distortions; especially in the "macro" range, this may be enough to make fusion less comfortable and/or to make the "window" edges appear less sharply drawn.  The same effect --again usually tolerable, but not always-- is noted in image-splitter stereo attachments for single-lens-reflex cameras. Gross departures from orthostereoscopic practice (say: the use of telephoto camera lenses) can produce "keystoning in depth," more properly called "frustum effect.

* Keystoning Distortion: This is an extension of the last entry and is based on a graphic revelation (below) and article ("The End of the Matter", Series 2, Number 27, page 4) by Editor Michael Gordon in the ISU's Stereoscopy. A glance at the below pair of stereograms should convince you that Mr. Gordon has settled the hash of many an old adversary on the issues of "to toe in, or not to toe in" one's camera stations, --in order to establish the stereo window. An extract follows:

"Curiously, the converged pair shows perspective but no increase in separation for the bottom (more distant) pole! If you stereo-view (fuse) the images, you will see the oddity of the vertical line passing through both poles at the same apparent distance.

The ISU globe at the bottom has both perspective and parallax changes showing that the bottom pole is indeed more distant than the top. If you stereo-view this pair, you will see that the parallel lines bisect the top pole, but not the bottom; this is correct behavior."

* Kromscop: (See Stereo World, Volume#15, #1) In the 1890s, Frederic Eugene Ives --a pioneer in printing processes-- developed an amazingly well thought out system of color stereography using black and white emulsions and compound transmission/reflection color filtering.  The Kromscop was the viewer and a self transposing stereo camera was also available.  The system used 3 color separation transparency pairs of images in registration to recreate a view called a "Kromogram."

* LEEP camera (system): A special camera that stereographed views of nearly 180 degrees of solid angle (2 pi steradians) by means of special aspherical fisheye lenses forming images on conventional film.  This very distorted swirl of an image pair was than viewed by the same kind of lenses to decode it into approximately the original world view.  A belt was supplied to strap it to a tree or vertical standard, thus avoiding a forward tripod leg or two getting into the view! Only three complete cameras and very few viewers were completed.  Not even the inventor/manufacturer --Eric Howlett-- owns one.

* Lenticular: (1) Of or pertaining to a lens.

(2) Shaped like a lens.  This word has many uses, several of which pertain to stereography: (a) A stereoscope which contains a lens --as opposed to one which used mirrors only-- was called by Brewster a "lenticular" stereoscope.  (b) A lenticular grid is an oversheet with refracting vertical stripes which may be used to view Autostereo (see) prints; (c) A lenticular screen (often "sliver"/aluminized) is a projection surface made up of tiny silvered concavities or simple vertical grooves.  This type of screen disperses the light better to the outside rows of the audience.  (A metallic coating also holds polarization of the light it reflects, allowing stereo projection by the opposite polarization of the two images and the two viewing lenses/filters.)

* Lenticular grid: See: "Autostereo".

* Lenticular Stereoscope: (1) Any stereoscope which contains lenses --as opposed to one which uses mirrors only. Sir David Brewster called his lensed stereoscope a "lenticular stereoscope" and accurately described the optical nature of such 'scopes, whether full-lensed or half-lensed "semilenticular" types. Such 'scopes are properly called "Brewster stereoscopes".

The "American", or "Parlor", or "Holmes" or "Holmes-Bates" stereoscope for 7 inch wide cards is, therefore, a "Brewster stereoscope", even though that term is is most commonly applied to the closed box 'scopes that Brewster designed. See: "Holmes, Oliver Wendell (Sr.)" and the "Holmes-Bates stereoscope".

* Level: The stereographic community has a fetish for level horizons, even when the horizon can't be seen.  To accomplish this, at least four stereo cameras (Kodak stereo 35, Revere 33, TDC Vivid and Wollensak) were equipped with spirit levels visible in the viewfinder.  It is readily possible to correct mildly tilted horizons when trimming print pairs by simply squaring to the horizon or a vertical.

* Lilliputianism: A stereographer's jargon term for minification through excess lateral parallax or "base."  See: "Hyperstereo", "Minification", "SILO effect".

* "Litho": Properly, this refers to lithography, but it's a commonly used term in reference to any mechanically printed image, such as the half-tone process.

* Loreo Camera: A very well thought-out design in which transposed pairs are placed onto a single 8 perforation (standard) 35mm film frame, such that they can be printed normally by any photofinishing machine. The film path is even bent, so as to preclude keystoning or depth-of field problems. Recent (2005) versions (sold by Berezin --see:

    http://www.berezin.com/3d/

--use good glass mirrors in place of the often unsatisfactory plastic molded ones.

* Marks, Gerald: Creator of fascinating anaglyphs and stereoscopic exhibits in San Francisco's "Exploratorium." Has been a visiting artist at the Massachusetts Institute of Technology.

* Mask: An opaque cover with apertures over a pair of stereo images.  The placement and size of the apertures determines the location in space of the "stereo window" (see) with respect to the subjects in the view.  Trimming a pair of stereo prints and mounting them appropriately will accomplish the same purposes.

* McKay, Herbert: Fellow of the Royal Photographic Society, author of Three Dimensional Photography, and founder of the Stereo Guild.

* Mer (milliertem): A metric method of dealing with the small angles of deviation (which see) found in a stereo view or in life.  A base of 65mm is assumed and the actual nearest and furthest distances in meters contained in a scene (in life) are each divided into 1000.  The smaller number (zero in the case of infinity) is subtracted from the larger to get the deviation in Mer.  The value for 4 feet to infinity is about 800 Mer. (See MiRF)

* Minification: Making something appear smaller through an optical effect (minifying lens [q.v.] or exaggerated lateral parallax [or "base"]).

* Minifying lens: A lens which diverges rather than converges light rays, with the result that objects viewed through it appear smaller ("minified," "minification").  Its strength is measured in negative diopters (see) and it's the opposite of a magnifying lens (magnification). Used in meniscus form for the correction of myopia (nearsightedness).  A minifying lens may also be used to correct a fixed-focus lens to a true infinity focus (usually minus -0.25 diopter is about right for this purpose), or to restore infinity focus when a lens has been fitted to a camera body thicker than that for which it was originally intended.

* MiRF (milli-reciprocal feet): Coined by Craig circa 1985 and proposed as the English measure equivalent of the Mer or 1000 divided by the distance in feet to the furthest subject minus the same for the nearest.  The distance from 4 feet to infinity contains 250M (normal IPD [q.v.] spacing assumed).  The deviation between 2 feet and 4 feet is also 250M, but if a hypostereo (see) base of 1-1/4 inch is used, the value reduces to 125M.

* Mirror viewer: See: "Wheatstone".

* Morgan, W.D.: Editor of the Stereo Realist Manual, 1954.

* Mount: A slide body or card used to secure, locate, and protect the two images of a stereo pair. Usually the term includes any mask involved.

* Mounting jig: A device for locating, holding, and sealing pairs of stereoscopic images into or onto a common mount.  Usually in reference to slide mounting, but also for mounting print pairs.

* National Stereoscopic Association: Headquartered in Columbus Ohio, the NSA is dedicated to the enjoyment, study, advancement and conservation of the history of stereoscopy and stereoscopic images.  It has sub-regions across the U.S. and has members around the world.  They are kept in touch by their excellent journal, Stereo World and have annual conventions.  See also: "Stereoscopic Society.

* NimsloTM: The brand name, taken from inventors Dr. Jerry Nims and Alien Lo, for a stereographic system that produces autostereo (see) prints having a very fine integral screen of vertical lens elements.  The camera has 4 lenses that make four of its 4-perforation-wide (and not quite full-frame high [=22 instead of 24 mml) frames at each exposure.  The camera is highly adaptable to standard stereography.  Recently, an inferior re-make (NishikaTM) has been introduced.

*  O.D.: Oculis dexter (right eye).  Often encountered as a technical abbreviation.

*  O.S.: Oculis sinister (left eye).  Often encountered as a technical abbreviation.

*  O.U.: Oculis uterque (each eye; both eyes).  Often encountered as a technical abbreviation.

*  O-Stereoscope: A special transparency stereoscope designed to view the 2x2 inch mounted split frames produced by a standard 8p planar camera fitted with a stereo attachment (sometimes called a "beam splitter" [q.v.l).  Since the attachment/lens system transposes the images on their way into the camera, the o-'scope need only "widen" the viewing path by means of prisms or mirrors.

* OO-Stereoscope: This 'scope is for 2x2 mounted split frame 8p slides similar to those made for use with the o-'scope, but for the case in which the camera was instead fitted with a miniature pair of lenses with parallel axes. In such a set-up, the slide in correct viewing orientation will have the left frame to the right, and the right frame to the left; thus, the frame halves have to be transposed by the oo-'scope.

* Occultation: The obscuring of an object --in whole or in part-- by another object in front of it (i.e. between the observer and the object).  For example, an eclipse of the sun is an occultation of the sun by the moon.  Occultation is a major part of the information used by the brain to determine distance: things are assumed to be farther away than the things which occult them.  One of the reasons that inverse stereo ("pseudostereo") (see) images are so uncomfortable to view is that the information derived from parallax and the information derived from occultation are precisely opposed to one another --parallax suggests that an object is close, yet it is occulted by objects which appear farther away.

* Olden/Tri-Delta (format, attachment): An interesting system that allows a standard 35mm camera to shoot stereo pairs onto a standard 8 perforation frame. The images are placed top-to-top and must be viewed or projected with optics similar to those used to make the transparencies. The camera aims skyward in use, so an accessory viewfinder --looking forward at the scene-- is part of the attachment.  Unlike mirrored attachments ("splitters"), the distortion introduced is not significant.  A 60-degree prism is used in conjunction with mirrors to fit the pair onto the 8p frame. (Compare to: "image splitter", "Elmo", "Savoye".)

 A method of dealing with deviation range (q.v.) in a view when using a non-standard camera lens base in stereography, either in hyperstereography or in close-up work.  (see: "Hyperstereography", "MiRF").  The closest subject matter in a scene should be 30 (or some figure between 15 and 50) times the separation of the camera lenses (or camera stations).  30 times is a value of 160 MiRF.  If the base is one 30th the film-to-subject distance, the apparent distance to the subject in the finished stereograph will be thirty times the normal base (which is roughly the average interpupillary distance, or 65 mm).  Thirty times 65mm is 1.95 meters, or 6.4 feet.  A corollary of this rule is the 3 degree rule for rotational displacement stereography.  Assume that the base-to-distance ratio is 1:30: that means that, if the photographic axes intersect at the given distance, they are converging at an angle of a bit under 2 degrees.  Thus, if one is using a turntable jig to rotate the object being photographed between the two exposures of a sequential stereogram, a rotation of 3 degrees or a bit less will yield a stereogram with similar stereo effect to that of one made by the one-in-thirty rule.  A 3 degree displacement gives stereo separation roughly equivalent to that of a base-to-distance ratio of 19.1, at the "hyperstereo" end of the comfortable viewing range. Staying under 3 degrees of displacement generally avoids shape distortion and images that are uncomfortable to fuse.

* Open stereoscope: See: "Holmes" and "Bates.

* Optical axis: A line through the optical center of a lens and perpendicular to the plane of its diameters.

* Optical center(s) (test for): A ray of light or a line of sight along the optical axis of a lens will pass through its optical center without undergoing any deflection or displacement. It will strike a screen or target beyond the lens as if the lens were not there (though a picosecond or so late).  See: "Collimation".

* Optical Transfer Function: A concept developed by Dr. Orville Becklund in analogy to electronic circuits for the mathematical treatment of optical systems.

* Optical Transposition: See "OO Stereoscope" and "Loreo" entries.
 

* Orthoscopic: When a stereoscopic image can be viewed with its planes of depth in proper sequence (as opposed to "inverse stereoscopic" (see).

* Orthostereoscopic: Such an image is usually understood to evidence all the requirements incumbent upon a "tautomorphic image" (see).

* Orthotopic: In the proper or conventional place. Transplant surgeons like this word; a liver transplant is usually "orthotopic," because the new liver is put in the same place that the old one used to be; a kidney transplant is "heterotopic," because the new one is usually more convenient to park in the pelvis.  In stereography, the term is usually employed to indicate that the right image is on the right, and the left image is on the left.  Images designed for reversing stereoscopes or for cross-eyed fusion would be heterotopic, while images designed for a Holmes 'scope would be orthotopic.  A confusing wrinkle in the use of this term is in split-frame stereo slides. In the "Hyponar" type of device (two lenses with parallel axes), the negatives or slide halves may be thought of as orthotopic with respect to the camera, because the left image is made in the left half of the camera's film aperture and the right image is made in the right half.  But because the lenses of course invert the image, the images are then heterotopic with respect to the viewer! When using these terms, it is wise to be careful to be sure that you aren't introducing more confusion than you're avoiding!

* Over-and-Under: See: "Under-and-Over" Formats.

* Panorama(stereo)gram: When photographic, a stereograph taken with more than two lenses or camera positions; thus affording more than one viewing position.  Nimslo lenticular prints are an example.

* Pantoscope: See: "Grapho(stereo)scope".

* Parallax: (1) The small disparities in alignments of elements at different distances from the observer in a  scene when one's viewpoint is shifted from one position to another (chronologic parallax); more relevantly to the topic at hand, the disparities from one eye to the other.

(2) The differences between any two viewpoints, as between the viewfinder and the taking lens of a non-SLR.

* Parallax Stereogram: An old autostereogram technique in which a vertical grid (say of wires, separated by their own diameter) is placed in front of a composite stereogram encoded into corresponding sequentially left-right-left-right thin vertical strips.  The placement is such that the wires block the right view to the left eye and vice-versa, allowing only a single fused view to be seen by an observer standing at the correct position.

* PePax: A jargon term for distortions resulting from non-standard perspective and/or parallax.  See discussion under "Claudet".

* Perforations: Film sprocket holes.  See discussion under "Format".

* Perspective correction: In planar photography, a geometrically adjustable camera (i.e.: "swings and tilts" in a view camera) or darkroom enlarger is used to "correct" lines in a photograph that appear to converge but are expected to "look" parallel in the photograph.  If, for classic instance, a simple camera is aimed upwards to take in the top of a building, the sides of the building will appear to be convergent toward the zenith.  A view camera would allow the photographer to simply raise the lens and/or tilt both lens and plate holder such that both would be plumb to the building --resulting in sides that remain parallel on the negative (and print).

Obviously, a wide angle lens (which must be used with a view camera also) on a small format camera can accomplish the same thing by simply shooting level and using only that portion of the film where the building is to be found.  The reason that converging building lines look "wrong" in a planar print is that the print is hung on a vertical wall for display and/or the observer assumes that the camera was aimed normally at the building.  It doesn't bother the observer that streets and train tracks in the photograph are seen to converge into the distance, because it's "understood" in our visual psychology that we stand on the earth and consequently have a vanishing point at the horizon (always assuming that we stare straight ahead --instead of upwards as the camera did).

However, a stereo view clearly shows us the attitude of the camera because the "window" (see) is (normally) square with and equidistant from the viewpoint of the observer. If the observer can be prevented from seeing the view as two planar images (as with a transparency viewer) before seeing it as a fused view, then upward or downward camera angles cease to create an image that looks "wrong" with respect to "perspective distortion." (Do you buy that?)

* Photogrammetry: A professional discipline which uses aerial photography (and other forms, especially in intelligence work) for ground measurements and cartographic work (topographic maps).  Stereo pairs, precision cameras, flight control, and fascinating instrumented viewing/plotting devices are employed.  For illustrative examples, anaglyphically printed, see Dieter Lorenz's Das Stereobild in Wissenschaft und Technik Deutsche Forschungs- und Versuchsanstalt für Luft- und Raumfahrt e.V. (2. Auflage), Köln, 1987.  (Available from Reel 3-D Enterprises (see). (Don't panic; it's bilingually printed in English and German!)

* Photographic axis: A line from the center of a camera's film aperture (i.e.: the "format" or frame on the film) through the optical center of the taking lens.  In a planar, non-adjustable camera, the photographic axis and the optical axis are the same. In a 5p 35mm stereo camera, the photographic axes of the two lenses usually converge at 6 to 7 feet from the camera, while the optical axes are parallel.  This is achieved by having the lenses each offset slightly toward the camera center; that is, each lens is placed a bit medial to the midpoint of the corresponding frame aperture. This creates a natural "stereo window" (q. v.) at that distance; that is, if the subject is more than seven feet from the camera, full-frame images will automatically be properly "windowed."  This was a great help to the mass commercial preparation of stereo slides, in that acceptable "windowing" could be obtained on most views with a standard mounting protocol --only for close-ups or for correction of horizon or "tilt" error was it necessary to crop in mounting.

* Planar (image): Flat, or in a single plane.  Thus, used to describe a two-dimensional or flat image, as opposed to a stereoscopic image.

* Planograph: See: "inverse" or "Pseudo" stereogram.

* Plasticity: The visual perception that objects have roundness and cross-section (as opposed to "cardboarding").

* Polarization (of light): In nature, certain crystals and angles of reflection preferentially reflect light such that it is divided into beams that contain light "vibrating" in one plane (vector) or another at 90 degrees rotation to each other.  Such light can be "detected" by inspecting it with another such crystal or technique.  When the vector passed by the second crystal is perpendicular to the inspected beam, it of course is seen (through the crystal) to darken.  By polarizing the light from two projectors in "opposite" (90 degrees from each other) vectors, and by providing the audience with glasses having similarly polarizing filters, even though both images fall on the same (special metallic) screen, the eyes see only their respective (left and right) images.  This is conveniently done with filters made of Polaroid brand film invented by Dr. Edward H. Land in 1932.  Rear projection onto ground glass also works well.

* Print: A vague term, but usually taken to mean a photographic as opposed to a mechanically reproduced image (in printing inks) in our circles; often used in contradistinction to "transparency" or "slide. See: "Litho".

* Prism: A wedge shaped optical element.  See: "Diopter".

* Prism stereo attachment: See: "Stereo attachment", "image Splitter", and "Elmo".

* Prismatic Lenses: (1) The "semi-lenticular" or "half lenses" of a stereoscope which have prism power by virtue of being used off axis (see "Diopter" etc.).

(2) See: "Angle lenses.

* Progression: The pattern by which film is advanced and the image frames are sequenced in a stereo camera.

* Projection masks: Are designed such that the centers of the apertures are at a constant distance (as opposed to --say-- the separation of points at infinity being held constant), thus the projected window always appears to be at the same distance with respect to the screen. Another approach is to simply let the black surround of the screen be the window (image runs off the screen sides), but the screen window might not be appropriate to the subject matter.

* Projection-viewer: A table-top device with a rear projection screen like the TDC (division of Bell and Howell) brand "Project-Or-View.

* Pseudoscope: See: "inverse stereoscopic".

* Pseudostereogram (sometimes "pseudostereoscopic"): (1) See: "inverse stereoscopic".

(2) The same image presented to each eye, perhaps with a "window" that appears in front of it, but otherwise having no depth (fake
stereograph/gram).

* Pulfrich, C.: Professor Pulfrich discovered visual phenomena and invented optical instruments.  He described the pseudo (and through cinematic and TV applications, real) stereoscopic effects produced by viewing moving images with one eye heavily filtered (delayed vision) now frequently used in (sort of) 3-D television broadcasts!

* Puppet theater effect: An apt description of the apparent reduced depth and size when the medium (such as a lenticular print (Nimslo] or table top rear screen projector) doesn't allow the use of visual infinity.  This is normally associated with "stretch effect" as well, since the image is further away from the observer than the original image was from the camera (allowing, of course, for any enlargement factor).

* Q-VU: Folding mask/mounts designed (with Craig) and marketed by Quentin Burke (817 East 8th street, Holtville, California 92250).  They're patterned after the TOA brand Japanese format folders but are made to U.S. standards and allow the user a choice of adhesives, window separation, styles, colors.  They do for prints what slide mounts do for transparencies.  Various methods of mounting prints into them have resulted in years of controversy.

* Random dot stereogram: See: "Julesz".

* Real image: One that can be caught on a screen (as opposed to a "virtual" image). See: "Actual image.

* Realist (camera): A 35mm stereo camera made by the David White Company and invented by Seton Rochwite. It uses what became British (3216:1960) and U.S. (ANSI-PH3.11-1953) standard 5 perforation format and Colardeau progression (see).  Many high-quality devices and services were offered under the Realist trademark.

* Reciprocity failure: Exposure or color error resulting from the use of films beyond the exposure-time range in which they are designed to perform well. This is occasionally a problem in the use of small apertures and fine grain films in stereography.  The usual trade-off between shutter speed and lens aperture is known as reciprocity; in general, if you double the length of time that the shutter is open, an equivalent exposure is obtained by closing the lens aperture by one 'stop" tie. multiplying the f-ratio by 1.41421 [that's the square root of two, of course, which is easy to remember because a 14-year-old thinks he's twice as smart as a 21-year-old.]).  Films fail to follow this trade-off without limit; at an extremely short or extremely long exposure time, there may be color drift or failure to achieve the expected negative (or slide) density.  For most photographic film, reciprocity is well maintained between about 1:2000th second and one second; at longer exposure times than one second, a longer exposure than that indicated by a meter is often required and shifts from neutral color rendering must also be anticipated and compensated for.  (The problem isn't the time per se, but the low light levels that obtain when such a long exposure is required. Emulsions have a "threshold" below which response is sluggish or nonexistent.  For special-purpose photography, there are tricks to overcome this, of which two are worth knowing about: [a] Technical emulsions are in some cases available in special formulations for longer exposure times --Vericolor PS, for example, is a Professional emulsion for Short exposure times ....the hard to find PL is a special-purpose film which has reciprocity in a longer exposure range; [b] It is possible to shift the threshold sensitivity of the film --to "hypersensitize" it --as for example by brief exposure to extreme cold [this is commonly used for astronomical photography, and pre-hypersensitized films may be purchased from many telescope dealers].)

* Red Wing Viewer (the): This stereoscope was designed and hand manufactured through the 1980s by Craig, then in cooperation with premier woodworker Luther Askeland (who perfected its design), and finally as a principal project of Luther Askeland with Aura Optics of Minnesota supplying finished lens pairs and Craig supplying finished brass parts. It was designed to be compatible with mainstream design standards of traditional stereoscopes and viewcards (following a survey of Keystone, Underwood, and other cards and stereoscopes). It appears to be the only standard print-pair stereoscope ever sold with published specifications --and for making new viewcards to use in it. See also the "Holmes-Bates Stereoscope".

* Reduced Base:  See the "Hypostereo" entry.

* Reel 3-D Enterprises, Inc.: A U.S. catalogue source of general stereographica.

* Relief: The appearance or fact of depth such as in carvings or stereography.

* Retinal rivalry: When a stereo pair are dissimilar, particularly with respect to density or positive/negative reversals, the eyes only allow the images to be seen one at a time in alternation.  Vision defects cause this too (differential color blindness, for instance).

* Reverse: The back side of a stereocard (or of any printed item). This term is less likely to provoke low humor than is the term, "backside."  The reverse of a card often bears an extended legend, or information concerning other offerings of the maker.  ("Verso" may be the preferred term.)

* Reverse(d) window: A term used to describe a stereo window (see) in which the margin conventions are reversed ...that is, the nearest object has a wider left margin in the left frame and a wider right margin in the right frame. Actually, the stereo window is not truly reversed; rather, it is positioned more distant from the observer than is the nearest object.  If that nearest object touches or crosses the frame margin, this may give rise to disconcerting effects that make the view considerably less comfortable to fuse.  The margin conventions are more fully described and charmingly illustrated in the entry, "Stereo window.

* Rising front: A camera lens panel which can be raised while the optical axis remains level and the film plane plumb.  See: "Perspective correction.

* Rotation (or "twist"): A difference between stereo pairs in which the horizons (whether or not present) are not level to each other.  This is more disturbing than straight vertical discrepancies.

* Rotational displacement stereography: A technique for sequential stereography (see) in which the displacement between the two planar images is angular rather than linear.  Either the camera or the subject may move.  In the former case, the camera may be mounted on an arm like the attached stool of some barber's chairs, and thus swing around the subject in an are.  In the latter case, the object itself is rotated, often on a turntable stage, while the camera remains stationary. In either case, the lighting must be very diffuse or the lights must remain in a constant orientation with respect to the subject; otherwise, the shadows will have noticeable differences between the two frames, and strange effects may result.  See: "Hammerschmidt jig" and "SLAWE effect" and "Umbus perpendicularis.

* Savoye format: A wide stereo format produced by a special attachment on a planar camera (still or cine).  The images are over and under on a standard frame, oriented bottom-to-bottom. C.f. "Elmo", "Olden/Tri-Delta", "image splitter".

* Scopeability: A jargon term used in reference to a view's over-all ease of being viewed. Standard separation of homologous points, suitable deviation, absence of rotation, a well placed window, balanced printing, accurate image alignment/masking/trimming --- all of these contribute to a good view.  This is sometimes spelled scopability, but to describe the difference as a point of controversy would be overstatement.

* Separation: See: "Base" and "Homologous distance.

* Septum: (1) The partition used in a stereo camera to separate the two lens to film paths.

(2) Any partition or design element that effectively separates the lines of sight of the eyes such that only their respective left and right
images are seen.

(3) The gap between the stereo pair of a print view.

(4) That which separates your left nostril from your right nostril.

* Sequential stereograph: A stereograph in which the two (or more) planar component images are not taken at the same time.  Rather, the camera is moved between the (usually) two exposures, or the subject is moved between the two exposures, or the spatial relations among the subjects (as in a diorama) are changed between the exposures. Obviously, when still cameras are used, this technique is suitable only for stationary objects --we specifically do not recommend it for small children or Springer Spaniels.  It can be used for moving objects, though, when the non-simultaneous images are alternating frames of a cinematic (movie) or video image.  Often a motorized camera is used to record sequential images of passing scenery as "dolly shots" on the ground or from an aeroplane.  Adjacent frames on the film can be used as stereo pairs (with slight discrepancies due to motions within the scene) if the distance between snaps is appropriate to the near point of the scene.  This technique can also be used in "Pulfrich effect" (see) stereo cinematography.  See: "Slide bar, "Shift technique", "Rotational displacement stereography", "Hammerschmidt jig".  Don't bother to see "Shift bar", as that entry will only refer you to "Slide bar".

* Shift bar: See:"Slide bar".

* Shift technique: A technique for taking sequential stereographs without special equipment. If one is using an ordinary planar camera such as a 35 mm SLR or "point-and-shoot," the first image is taken with the body's weight on/over one foot.  The second photograph is the taken after simply shifting weight to the other foot.  The amount of lateral displacement caused by such a weight shift is generally within the range that will give a satisfactory stereograph.  This is sometimes called the "astronaut shuffle," because it was occasionally used by NASA astronauts to make stereographs on space missions. Those of us who learned it from our stereographer fathers long before there were such things as astronauts eschew this term as an anachronism.

* SILO effect (an Acronym for Smaller In, Larger Out, brought to our attention by Dr. Fred Weitz [SSAB#848], an optometrist stereographer): Minification by exaggerated lateral parallax. In views made in the "hyperstereo" range (see), the object depicted often appears smaller than it really should.  This is sometimes called by stereographers' jargon terms such as "Lilliputianism"; the optical effect of reduced apparent size is technically known as "minification" --- the direct opposite of "magnification."  (It is sometimes also called "miniaturization"; this term more properly refers to the actual rather than the apparent reduction in size of something.)  The most familiar examples of minification are the effect of looking backwards through an ordinary pair of binoculars, and the size shift on viewing through corrective lenses for nearsightedness.

The minification seen in hyperstereo is particularly interesting, because it does not result from a change in the image size; rather, it results from excess parallax, from the wide spacing between the two taking lenses.  This yields a discrepancy between two sets of input to the brain's cortex.  If the image is a familiar one, there are similar images in the brain's library of remembered images, and size and distance estimates will be made from simply comparing the planar image to those remembered ones.  The degree of difference between the two planar images of the stereograph, then, is such that the image looks much closer --that is, there is a greater degree of apparent convergence than would fit with the distance estimate made from the planar view alone.

To anthropomorphize a bit (if indeed one may call it anthropomorphization to liken the human brain to a human), the brain may then be thought of as trying to reconcile these two discrepant distance estimates.  One resolution is to conclude that the eyes are suddenly much farther apart (and the object is as far away and as big as you'd guess from its planar appearance); another is to conclude that the object is really smaller than would be guessed from the planar view (and really is as close as the stereo effect would suggest).  The former resolution fits no previous experience or established anatomical connections, while the latter will have experiential precedent (everybody's seen doll houses, for example).  The brain usually chooses the latter, or makes a compromise, and perceives the depicted object as smaller than it really is. If the object pictured is not a familiar one, the brain just can't decide how big it is.  (The acronym "SILO effect" refers to the handling of the image at the cortex.)  Minification through parallax effect is also termed "convergence micropsia.

* SLAWE effect: Acronym for "Shadows Look Awfully Weird, Eh?  A Manitoban variation on Umbus Perpendicularis Shiftus ---OOPS (see).

* Slide bar: A device for taking sequential stereo pairs for still life stereography. A planar camera is shiftedhorizontally between exposures.  The optical axes for both exposures should be parallel, but are often "toed-in" to conveniently create a "window."  Such toe-in will be acceptable if not too extreme; one must use great caution, however, in that it may produce enough keystoning (see) to be a problem, and may increase the background discordance. Commercial slide bars are readily available and of excellent quality; however, because they are most often manufactured and sold for other purposes, your friendly neighborhood photo dealer may stock quite an assortment without even knowing it.  Some of the best are actually marketed as fine-positioning rails for copystands; but they are perfectly happy to sit on tripods instead of copystands, and they provide a calibrated rack-and-pinion track with a locking screw. An example of a good quality device for under $50 (1990 price) is the "Quadrail" positioning bar sold by Spiratone, a mail-order photo supplier in Flushing, New York.

* SMPTE: The Society of Motion Picture and Television Engineers determines and lists standards for equipment, formats, and techniques of imaging systems.

* Snap 3D Camera: {Entry pending --classified information as of March, 2004]

* Social Animal ("Man is a ---"): Baruch Benedict Spinoza, 1632 - 1677, persecuted free thinking philosopher, heretic, and lens grinder who managed to get himself excommunicated as a Jew and stated: "the most tyranical governments are those which make crimes of opinions, for everyone has an inalienable right to his thoughts".

* Social Stereoscope (The ___): Dissertation by Marilyn Faye Morton, Emory University, 1998. The Social Stereoscope "examines the field of stereography --the three dimensional variant of the photographic medium --that set a new standard of pictorial truth for a virtually naive consumer base in the mid-nineteenth century."

* Space control: A technique of trick photography in which multiple exposures or printings insinuate one image into another such that they appear to share the same space.

* Space image: An image that appears to be solid with scene elements at varying distances from the observer.  A space image, whether still or cinematic, is "static" in that only two fixed perspectives and one fixed set of parallax values for the scene elements are available, as opposed to a scene in life or the reconstructed light wavefront from it offered by a holographic image.  Both types of space image are very distinct from the simple "actual image" (see) or "real image" which can be projected onto a flat screen and/or treated as such.

Note: We know of no term for a holographic or life image, except stated as such.  The term "Dynamic Space image" (again, either still or cinematic) seems to be a possibility.  An integrated multi-station stereo view such as an XogramTM or NimsloTM lenticular print occupies a middle ground in that if offers a series of fixed perspectives, simulating a "life" or "dynamic space" image.

* Spectacles: In stereoscopy: glasses worn for the purpose of decoding a composite stereogram, projected or directly viewed.

* Spinoza --see: "Social Animal".

* Split frame format: Usually, a stereo pair recorded on a single frame of a planar camera's film by use of a stereo attachment.  May be side-by-side or over-and-under.

* Split stereogram: Not really a complete stereogram in that the subject matter of each of the pair is supplemental to the other.  Usually done as a stunt.

* Squeeze: (1) The opposite effect of "stretch." The perceived flattening effect caused by walking towards a projected stereo composite image or viewing (magnifying) the image (side-by-side) pair with ever shorter focus ocular lenses.  This occurs when the use of long telephoto lenses are attempted for stereography.  You then have to use equally long viewing lenses to make the view "'scope" right (thus: defeating your purpose).  See: "Stereo telephotography."

(2) Methanol, especially that mechanically extracted from jellied methanol (e.g. SternoTM).  The folk etymology is probably accurate, here; the name derives from the practice of forcing the jellied methanol through a nylon stocking to separate the methanol from the semi-solid matrix.

* Standards: See: "ASA," "SMPTE," and "ANSI."

* Stereo: From the Greek word "stereos," meaning solid.  Thus, a term or prefix referring to processes which add depth.  Stereophonic sound is sound which allows depth to be perceived, as Stereography is photography in which depth may be perceived.

* Stereo acuity: The ability to distinguish different planes of depth, which depends on the smallest angular differences of parallax that can be resolved binocularly. Under laboratory conditions (and --probably-- with visually large targets), claims have been made for the detection of parallax differences as slight as 3 seconds of arc!  For practical stereography, a value of 1 minute of are (the same as average [20/20 vision] detail resolution) might be best assumed.  This value determines the maximum distance of "stereo infinity" (which see).

* Stereo attachment: A prism or mirror device for taking stereo photographs with a planar camera.  The common types split the frame into two side-by-side images.  Another splits the frame horizontally (see: "Savoye").  See also: "Twin lens attachment".  See discussion and illustration under: "image splitter."

* Stereo Base: See "Base".

* Stereo diplopia: See: "Dynamic stereo diplopia".

* Stereo Guild: An international association of stereographers that circulated 35mm stereo slides among the members.  See: "Herbert McKay".

* Stereo infinity: The farthest distance at which stereo effects may be seen. If it is assumed that the smallest resolvable angular difference in parallax between the eyes is 1 minute of are (.0003 radian), then one should not be able to distinguish any depth beyond scene elements that are about 700 feet distant (at least not without moving one's head from side to side to increase the parallax).  The actual average limit (threshold) of resolvable parallax is now considered to be about 6 seconds of are for a stereo infinity of 7000 feet(!) but that's hard to capture in a photographic stereo pair.

* Stereo macrography: Stereo photography in the "macro" range. As in planar photography, this term is generally used to describe images in the range wherein the film image is approximately life-sized.  In other words, stereo macrography is the stereography of subjects such that the finished view presents a size magnification of =1x (as compared to what would be seen in life of the subject at 10 inches [25.4 cm] distance) to 10x.  See: "Stereo magnification.

* Stereo magnification: When a stereo camera with lenses of the approximate separation of human eyes is used to make a stereograph, when the stereograph is viewed at film image size through stereoscope lenses of the same focal length as the camera's, then the stereo magnification is considered to be unity (one x).  If the camera lens base is doubled, the stereo magnification is then 2x.  Because of the need to keep the "deviation range" of a view within limits, stereographs that contain subject matter both distant and nearer than 5 feet (or so) are either avoided or taken with reduced lens separation, which is stereo demagnification".

**Don't confuse stereo demagnification ("hypostereo") with its main effect: subject magnification by decreased parallax.  Stereo magnification ("hyperstereo") causes subject "minification" by means of increased parallax. These parallax effects are considered separately from any actual planer subject magnification in the view.

* Stereo micrography: Stereo macrography of a more extreme degree. The prefix micro- is often applied in both stereo and planar photography to images with magnification that is in the range of a microscope (such as a dissecting 'scope) rather than being just close-ups in which the film-plane image is as big as the object being photographed. Customarily, the prefix macro- is applied up to about 10 magnification, and micro- is applied for magnifications greater than 1Ox.  Ideally, stereo demagnification tracks size magnification at about 7 to 1 for "scopeability."  A size magnification of 10x (i.e.: camera distance is 1 inch to subject) would use an equivalent camera base reduction of 1/70th the normal 65mm to 70mm.

* Stereo projector: A device (which might be an unadapted planar projector in the case of anaglyphic slides) for projecting stereograms upon a screen.

* Stereo radiogram (or radiograph): An X-ray stereogram Typically, these are made to determine whether a "coin lesion" (a small, round, shadow suspicious for tumor) is within the substance of the lung or not.  A standard chest X-ray is made, then the patient is rotated a few degrees and the process is repeated.  Since the advent of computerized axial tomography ("CAT scans") (which produces X-ray or magnetic  resonance images looking like salami slices) the stereo technique has seen far less used; it is still sometimes used in rural communities, where access to CAT may be limited.  A stereoradiogram may also be called a Stereorröntgenogram or a Stereoskiagram (Roentgen [or: Röntgen] was of course the discoverer of X-rays, and skia is old Greek for shadow).

(It was Dale's painful experience with a broken collar bone which delivered him to the mercies of a by-the-book X-ray Technician --who murderously (of the X-ray plate pair) rotated poor Dale to accomplish an old fashioned stereoradiogram.  Much more information could be viewed in such pairs if technicians used displacement of the X-ray cavity/source instead. Anyone who's worked with the old and delightful technique of rear-projected shadow anaglyphy will immediately appreciate my opinion.  See the new ['97] entry "Keystoning Distortion" for more discussion. --CFD)

* Stereo telephotography: The theory and making of stereographs with longer than normal camera lenses, which isn't simply the inverse of stereomacrography.  It is better thought of as the opposite of wide angle (i.e.: short focal length) stereography.  Wide/short camera lenses result in views that --when viewed in a normal 'scope, suffer from "stretch," due to the disparity between taking and viewing focal lengths.  The opposite occurs ("flattening", or "squeeze" [see) when taking lenses are much longer than the viewing lenses.  Resorting to a wide base (say, two synchronized SLR cameras overlooking a cityscape from a cliff) to make up for the "squeeze" introduces apparent geometric distortions in the subjects (ie: the back of a passenger bus seems suitably further away, but it's just as big as the front [!]; therefore [saith the perplexed brain], the cross section of the bus must enlarge toward the rear end.).  This might properly be called "frustum effect" (see).

* Stereo telescope: Invented by Hermann von Helmholtz, this is a binocular optical device that magnifies depth (stereo) instead of (or, as well as) size.  Used for gunnery and other ranging equipment; if one or more of the mirrors or prisms can be rotated through a measured angle, the angle of convergence for fusion can readily be determined and the distance to the object inferred with reasonable accuracy.  Simply put, this is an overgrown version of a camera rangefinder. Also called (by the inventor): a "telestereoscope."  See "Claudet" and "Minification."  [image]

* Stereo typogram: A stereogram produced by shifting otherwise identical type between a left and right pair.

* Stereo views: Any stereogram, but often used more specifically to refer to the standard American format stereocard, 3.5 x 7 inches in size.

When viewing through a window, the right eye sees a bit more to the left, and the left eye sees a bit more to the right. Mounting stereographs correspondingly creates an apparent "window," through which one views the subject.  [image]
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The most comfortably-viewed stereographs are ones in which all objects appear to be on the far side of the "window," with the occasional exception of an object which may appear to pierce the plane of the window other than at the edges.
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As you can perhaps appreciate from the above, an object on the far side of the "window" will appear to be more to the right in the right-hand image (that is, have a wider margin on its left side) and more to the left in the left-hand image (have a wider right-hand margin).  Just the opposite is true in an object which is closer to the observer than the "window":
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And finally, if somewhat obviously, an object which is in the plane of the "window" will be identically positioned in both frames:

Ordinary practice would be to have all images in the view appear as though they are beyond the plane of the "window." It may occasionally be effective to have an object poke through the plane of the "window"; that's not distracting if it is something which physically could do so in a real window.  But if it's not "physiologic," it can be very distracting.  For example, a person should not normally appear to have his head cut off by the frame of the "window," such that it seems to float in disembodied splendor in the middle of your stereoscope!

A special case worth mentioning is the use of windows with arched tops. Because the arches are in part vertical, they give distance information. Therefore, if one uses the upper edge to align the homologous points in a print, there is a great danger of error; homologous points should be the same distance below the top edge (of a curved top) only if the object is meant to be in the plane of the "window."  This may be seen in the illustration below. The caneweave-patterned blob is in the plane of the "window," and is identically positioned in each frame.  The speckled blob is behind the "window," and is slightly further down from the top edge in the right-hand "chip."  But it's the same distance up from the bottom of the image in each "chip."  It is best for arched tops to make all measurements from the bottom, and with great care.

* Stereo window(camera): A stereo camera may be constructed to be "self-windowing" without keystoning by having its photographic (camera) axes converge, even while its optical axes are parallel.  This is achieved by having the lenses offset from the center of the film aperture.  See illustration under "Photographic axis".

* Stereo window (viewer): In certain stereoscopes such as the Keystone Telebinocular, the "window" is a physical rather than a virtual thing: A mask determines the stereo window and it is sometimes referred to as such.

* Stereogram: The artificial 3 dimensional image as a whole. It may consist of a side-by-side pair, a coded composite, or a sequence that is coded by persistence of vision.

* Stereogrammetry: See:"Photogrammetry".

* Stereograph(y): (1) A photographic stereogram and the making of such.

(2) (now obsolete): the art and practice of solid drawing.  See "Tamari, V".

* Stereographoscope: A heavy table-mounted viewer that carries a large monocular lens above the two regular stereoscope lenses.  The tongue (or stage track) slopes downward and carries a sliding stage that can hold large planar photographs as well as stereographs.  Sometimes called a pantoscope.

* Stereophoroscope: A calibrated stereoscope for the viewing of calibrated stereo images, used for the measurement of stereopsis, exophoria, etc.; e.g. the Ameson Stereopsimeter.

* Stereopsis: The physiological and mental phenomenon of converting binocular vision parallax differences into the sensation and awareness of depth; a "space image" in the cyclopean center or mind's eye.  See: "Dynamic stereo diplopia."

* Stereopticon: A troublesome word, which in common parlance has come to denote --inaccurately-- a stereoscope. The Oxford English Dictionary cites Knight's Dictionary of Mechanics as the earliest appearance its researchers found in print (1875); in that instance, it referred to a dissolving twin-image "magic lantern,' which could be used to convey information about depth by the blended sequential presentation of a series of planar views of a subject. Such presentations, while perhaps dramatic to the audience of a century and more ago, were not stereographic.

The word was also applied to less fancy, single "magic lantern" transparency projectors, such as those used to project song slides in the nickelodeon days.  Stereopticon is said also to have been a commercial name for a traditional non-reflecting opaque projector which allowed the projection of a printed image (say a photograph or a post card picture) onto a screen by means of a large lens system and intense light (and heat!).  The word Stereopticon has nothing to do with stereography except for its all-too-common misapplication to various stereoscopic devices.

Unfortunately... two of those misapplications have been by manufacturers of stereoscopes!  The earliest (that we're aware of) was Rawson's "Stereopticon" of 1867 (see: Stereo World, March/April of 1980).  The latest is the current "Stereopticon 707" by the Taylor-Merchant Corporation (a cardboard folding viewing device).  And perhaps even more unfortunately...the usually-scholarly Smithsonian Institution has (May, 1990) Holmes-type stereoscopes on display, bearing labels identifying them as Stereopticons!

* Stereoscope: (Usually) a lensed, prismed, or mirrored device for fusing and viewing stereo pairs of images. See "Lenticular Stereoscope".

* Stereoscopic Society: An international association of branches and individuals dedicated to the practice and advancement of stereoscopy.  The noble American Branch is in association with the National Stereoscopic Association; in 1990, recognizing that formal ties with the parent British Society no longer existed, the American Branch officially changed its name to the Stereoscopic Society of America.

* Stereoscopic: Means "solid looking." May refer to any experience or device that's associated with binocular depth perception.

* Stereoscopic base: See "Base" entry.

* Stereoscopic projection: Traditionally refers to projecting a stereo pair onto a large screen such that many can see the images.  Various means have been used to "code" the images at the projector(s), then decode the pair again --respectively to the left and right eyes of onlookers.  See "Eclipse", "Polarization", and "Anaglyph" entries. Projection might also refer to virtual optical, direct retinal scanning, reflex display systems, and holographic techniques.

* Stereoscopic Reflector: See "Beam splitter" entry.

* Stereoscopic synthesis: (1) Same as "stereopsis" (q.v.).

(2) Refers to research evidence (using random dot stereograms) that stereopsis is somewhat of an independent "6th sense" (after balance, touch, taste, etc.).  There may be a region of the brain dedicated to processing and analyzing the parallax differences between the perspectives of one's left and right eyed vision.

* Stereosynthesis: At this late date (8/6/06), Craig (co-author of Glossary) has finally learned (by happening across it during a Google search) of an earlier coinage plus trademark usage of the term: "StereoSynthesis" by David M. Geshwind. Geshwind is the fellow who figured out how to colorize early black & white movies (1986 patent process) using computer assisted editing techniques --which are more like human-assisted computer techniques. He realized that this same system could be adapted to track and assign parallax information to image objects, demonstrating and patenting this process in 1990.

As president of 3DMG, Geshwind has recently licensed the use of his process to IMax for the conversion of segments of "Superman Returns" into 3D.

Craig doing business as "StereoType" in Florence, Oregon, came up with this same term as a name for his conversion of 2D commercial art to "in the round" photo-realistic stereo pairs. He planned out the conversion with hand drawings, trigonometry, and homemade charts, then executed it by using the excellent general purpose graphics program "Picture Publisher-3.1", which was developed by Micrografx for PC computers.

* Stop: (1) Usually refers to the "f-stop" of a camera lens, which is the numerical value of its effective focal length divided by the adjusted diameter of its lens aperture.  Usually, these numbers are expressed in even multiples of the square root of 2 (1.41421): f:1.4, f:2, f:2.8, f:4, f:5.6, etc. It is denoted (e.g.) f:5.6 because the effective size of the opening is the ratio between f (the nominal focal length) and the f-stop number (5.6 in this example).  The "effective" focal length is generally the distance from the nodal point of the lens to the filmplane; it's equal to the lens's nominal focal length when it is focused at infinity.  At extremely close distances ("macro" range), the difference between nominal focal length and effective focal length may become great enough to require correcting the exposure --that is, the lens is far enough from the film that the "distance" part of the ratio cannot be assumed to be close enough to the focal length to make no difference.  The "adjusted" diameter is a term that is used because --depending on the design of the lens and the precise location of the iris diaphragm ---the actual diameter of the opening in the iris may not give accurate exposure information. It may behave as an opening of a slightly different size.

(2) An increment of exposure that halves or doubles the amount of light reaching the film.  For example, a one-stop overexposure is made by opening the lens aperture by one step, or by doubling the time that the shutter is open. The light passed is doubled by increasing the diameter of the lens aperture by the square root of 2.

(3) Any aperture which controls light or line of sight.

* Stretch: results from viewing an image pair from an effective distance greater than that of the camera's distance from lens to film.  This occurs when the focal length of a viewer's lenses is significantly longer than the focal length of the camera in the case of transparencies.  For prints, the viewer focal length can increase over that of the camera's in direct proportion to the enlargement of the print (cropping has no effect).  It is common for a stereoscopic system (ViewMaster, Realist, etc.) to incorporate a stretch factor of 1.4 or so, as well as some hyperstereo effect (see).

Note: This presents a problem in using the much sought after wide angle (i.e.: short focal length) lenses for stereo photography.  See also "Stereo telephotography" for the opposite problem of resultant "squeeze.

* Supplementary lens(es): See: "Accessory lenses.

* Table viewer: (1) See: "Projection viewer".

(2) See: "Graphoscope".

* Tamari (Vladimer): is a Palestinian artist and inventor (now living and working in Japan, apparently) who feels that the longstanding practice of "flattening" our real world onto surfaces for its representation has made us culturally "space blind": unable to conceptualize three-dimensional space realistically.  He has invented (1963)† a mechanical instrument for direct stereoscopic creative drawing in the round.  It consists of a held stylus ("space pen") that couples to two pens moving on two sketch pads which are viewed stereographically by the artist.  Downward motion of the stylus causes the pencils to separate slightly, thus creating the wider parallax required for an illusion of depth.

The pencils otherwise follow the freehand motion of the stylus in the X & Y coördinates. Tamari points out to us that this isn't simply a method of creating 3-D drawings, but a new method of drawing per se.  He has developed accessories that include: a spherical compass, a universally orientable plane, an axonometric perspective 3-D template system, and an optical method for visual projection of 3-D drawings over real space (to "see" one's drawings superimposed in actual visual space superimposed on 1:1 scale with objects seen at near or vast distances).  The architect, for example, can sketch cos (q.v.) building upon the vacant building site.

Tamari's drawings are, of course, breathtakingly beautiful to befuse. One can imagine a fully perfected computer graphics workstation version of Tamari's invention: --which allows the artist to (say) command a cartoon character to turn around in order to complete its other side and then enter the complete beast for further reference and manipulation/progressions. An X-Y-Z 3-D electronic brush in the right hand, a joystick in the left that controls a spectrum of color left & right while setting shade/tint to & fro. An index finger pressure trigger on the right (held-like-a-pencil) could control "tweening", etc. features from on-screen menus.
(CFD -March, 1990)

†Japanese Patent #762196,50.3.24. Vladimer Tamari, 3DD Co. Ltd., 1-36-16-101 Sakurahimmachi Setagaya, Tokyo, Japan.  Tel. 704-1269.

* Tautomorphic image: A stereoscopic view that presents the original scene to the observer exactly as co would have perceived it in life; with the same apparent scale, positions of scene elements, and a stereo magnification of one for all subject matter in the view. Many familiar stereoscopic systems use "stretch" and "hyper" factors of 1.4 or more.  Herbert McKay and J. G. Ferwerda consider such departures from ortho (say: between 0.7 and 1.4 [there's that square root of 2 again, cropping up everywhere]) to be unnoticeable.  (The credibility of those who complain about lack of depth in 55.5-mm-base Nimslo pairs is challenged here.)

* Telebinocular: Keystone's brand name for a stereoscope with built-in window masking, producing results similar to that of viewing a stereo slide (in that your world view contains only the view, surrounded by blackness).  The 'scope only works well with views made to Keystone's standards of format (adjacent print pairs, 3 inches wide, and floating separation of homologous points at infinity [anything from 3 to 3-3/8 inches] to keep the print format constant between normal and close-up views).  The deluxe versions came with a hazardous electric lamp that worked poorly with curved views.

* Telestereoscope: See "Stereotelescope".

* Three-degree rule: When using rotational displacement between views, a 2° shift is roughly equivalent to a base-to-distance ratio of 30 (actually 1.909"), and a 3° shift is roughly equivalent to a ratio of 20 (actually 2.862"). Thus 3° is at the "hyperstereo" end of the routinely comfortable viewing range, and exceeding 3° displacement may introduce noticeable shape distortion or may result in an image which is uncomfortable to fuse.  It is therefore a useful rule of thumb to work at two to three degrees angular displacement when using turntable techniques.  See: "Hammerschmidt Jig," "One-in-thirty rule, "Rotational displacement stereography."

* Three-dimensional or "3-D": The pitch man's way to say "stereoscopic".  Derived from René Descartes' system of coördinates, which assigns two dimensions to a plane (usually denoted x and y), and three dimensions (x, y, and z) to a solid space.

* Tilt: (1) Non-horizontal orientation of the horizon in a stereo view. A stereographic view in which the horizon (whether or not present) is not level to the format is thus said to be tilted.

(2) The rocking of the lensboard of a camera (or enlarger) about a left<--->right axis, used for example to correct keystoning distortion (q.v.) in photographing buildings.

* Tilting stage: Used in microscopy to produce a sequential stereo pair.  Sometimes a "rocker" or "teeter-totter" setup is used in which a pin is placed laterally under the slide at the optical axis, the diameter of which is no more than about 1/50th the slide's length --producing about 2.3 degrees rotation of the (top illuminated) subject.

* Tissue: A translucent print stereograph, often tinted and of French origin.

* Toe-in: The creation of a stereo window when using planar cameras by the expedient of aiming their optical axes to converge at the desired window distance.  See the "keystoning" discussion.

* Transposition: Reversal of the two images of a stereo pair. Because lenses produce inverted images, stereo cameras produce a stereo pair that are reversed, left for right, if viewed in otherwise-proper orientation on the film. If the pair is on a single piece of film, the images must be transposed by (1) cutting them apart and swapping them,

(2) using a transposing viewer,

(3) using crossed free vision,

(4) using a LoreoTM or ArgusTM stereo camera.  (Sorry, a mirror won't work.)

* Tri-Delta: See: "Olden/Tri-Delta".

* Tru-Vue: The Tru-Vue company preceded, parallelled, and then was bought-out by ViewMaster.  It was the first U.S. company to distribute views on 35mm film (strips) and its titles included many fine travelogues. Non-transposing viewers were marketed to use with the film strips.

* Turn-table Stereography: See discussions under "Base", "Toe-in", and "Keystoning"..

* Twin lens attachment: A pair of small lenses on the same lens "board" that fit a planar camera with interchangeable lenses such as a 35mm SLR. See: "Split frame" and "Format."

* Umbus Perpindicularis Shiftus effect (or "UPS effect" and popularly called "OOPS!"): Thanks to NSA member Rob (tongue-in-cheek) Oechsle of Okinawa, we now have a proper definition of this vexing phenomenon: "Stereo photographs produced by the shifting of a single camera with an ATTACHED light strobe will produce shadows perpendicular to the axis of depth, emanating in a flat plane from each solid object or point causing the shadow." Or in other words: "This is caused by the constant lens/strobe/object angle that is maintained without regard to the changing position of the camera.  The shadows will always be in flat sideways protrusion from --and in the plane of-- the objects they are cast from.

* Under-and-Over formats: These are systems in which the left and right pair are presented above and below each other, either as formatted on a film frame (see stereo attachments), matted together as a pair of prints, or scanned as a top and bottom separated pair on a television screen (to be viewed with prismatic glasses, such as the "KMQ" brand viewers).  Generally, a mirrored or prism viewing device is used to enable the observer to fuse the pair.  These systems lend themselves to images with wide "aspect ratios" (image width can far exceed height) whereas side-by-side systems lend themselves to tall aspect ratios. Such a format is sometimes used in the gallery display of stereographs, as it may be used for views of almost any size --one merely views larger pairs from a greater distance.

* Verso: See "Reverse".

* Vectograph: A polarization coded stereoscopic transparency that is viewed "directly" with polarized glasses. An invention from Dr. Land's laboratories.

* Viewer: (1) A vague term that, among stereoscopists, refers to any device that either decodes (glasses) or allows fusion (stereoscope) of stereograms.

(2) The person viewing a stereograph; for clarity, it is probably better to call the person the observer rather than the viewer.

* ViewMaster^tm: A system of stereo photography, characterized by small transparency pairs mounted in a flat, circular reel.  Originally a product of the Sawyer Company, it remains the only mass marketed system (at this writing) for viewing stereograms that penetrates the U.S. market.  Its familiar reels holding 7 pairs of little transparencies enjoy over 33% market survey product recognition.  The product line in the United States once included a very high quality ("Personal") camera and many beautifully photographed reels depicting travel, natural wonders, science and engineering themes, traditional children's stories, and significant events.  Good reels are still obtainable from importers (and collectors/dealers), but the current reel production is mostly cartoon pap.  Sources as of early 1993: Charley van Pelt and Associates, 1424 East Mountain Street, Glendale, California 91207. Worldwide Slides, 7427 Washburn Avenue South, Minneapolis, Minnesota 55423.  See also ads and listings by NSA collectors and dealers.

* Vignetting: Darkening or cutting off of an image as one gets farther from the center; this may be done deliberately to create a mood, or it may be an image flaw. This is only peculiar to stereography in that certain brands and models of stereo cameras are known to shade off the corners of their film format at small apertures (Realist and Revere Cameras with 1:3.5 lenses).

* Virtual image: See discussion under "Actual image".

* Waack, Fritz G.: Author of Stereo Photography (translated into English from the original German and published in 1985; English version edited by Susan Pinsky and David Starkman of Reel 3-D Enterprises, who also distribute it).

* Waterhouse stops: Lens apertures which are not variable, usually consisting of simple holes in a metal plate, which is either placed before the lens or slid between its elements.  Many inexpensive cameras use Waterhouse stops to control exposure; the film speed selector inserts such stops either into the light path to the photosensor, or into the photographic light path.  When John Waterhouse introduced these in 1858, he probably didn't envision their application to anaglyphic stereography (a pretty safe bet).  You can apply pairs of small (say: f/16) apertures to the front of a modern SLR lens (50+mm and f/1.4, say), cover them with red and blue-green filters and do room-scale or close-up anaglyph stereography using standard films and processing.  Make the blue aperture half again as large as the red.

* Whack: The brand (or imprint) of the (claimed) first U.S. anaglyphic cartoon comic book.  The editor said: "Let's give it a 'whack'!"  (Not to be confused with "Waack, Fritz G." --or with anything else, for that matter).

* Wheatstone, Charles: The inventor (1838) of the stereoscope that bears his name (and the word "stereoscope"; and the Wheatstone bridge, useful for measuring electrical capacitance and/or resistance).  The Wheatstone 'scope fuses a stereo pair of images with the aid of mirrors or reflecting prisms.  This was the first stereoscope (if you discount F. A. Elliot's 1837 eye-scrootcher) and was invented before photography.  Amazingly (and several years of cybernated ancient literature searches confirm): Wheatstone was the first person to appreciate the nature and publish the principles of stereo vision and stereoscopy.

* Window: See: "Stereo window" and "Mask".

* Witness notches: The bottom edges of the film plane apertures in stereo cameras usually bear notches that identify left from right frames and what brand and model of camera was used.  The Nimslo camera --oddly-- has instead a witness "tooth" in one of its versions (invert the camera when you file it off --to keep filings from getting into the works).

* Wodniw: A slang term for "reversed window, (see) the circumstance in which the "stereo window" (see) is located to the far side of the nearest object, with distracting consequences.

* Xography^TM: A proprietary name given to a lenticular autostereo print view system that works very much like Nimslo^TM prints.

This glossary first appeared in the Viewsletter / of the Stereoscopic Society and was without any copyright from the original version's author: Craig.  This very much improved edition owes everything to Dale for its motivation and illustrations plus a large debt to his contributions of new items and rewriting of old ones for clarity and form.  It has also benefited greatly from the recent (and excellent!) book: The World of 3-D by Jacobus G. Ferwerda.  My original knowledge of stereoscopy was gained from Herbert McKay's Three Dimensional Photography (the influence of which should be evident).  The  first basis and inspiration for Glossary
was W.C. Dalgoutte's Glossary, a revision of which appeared in "Technical Supplements #s 21 and 22" to Stereoscopy, journal of the International Stereoscopic Union.  Several references are also adapted from articles by David Starkman and Susan Pinsky that appeared in their (deceased) magazine: Reel 3-D News (thanks!). {CFD}

In my own defense, I note that my main contribution was to offer the use of my Macintosh^TM small computer and laserprinter to enhance the typography when Craig mentioned he had been thinking of re-issuing the glossary. Because our computers were incompatible, Craig "uploaded" the text in segments to the CompuServe^TM electronic mail service, from whence I "downloaded" it.  It was then converted to PostScipt^TM laserfonts; a little editing was done here and there, some to clarify and some to add curmudgeonly and pedantic asides; a few tongue-in-cheek definitions and comments were added; a few illustrations were added, and a camera-ready laserprint was prepared.  --{D.E.H.}

Now that the opus is on Macintosh disk, it can readily be revised and updated. If anyone wants to add to this listing, or refine what's already there, feel free to contact either one of us.  We'll be happy to credit our sources in the text, but expect the same generous attitude toward copyright that we have adopted (see below).  Furthermore, anyone who'd like a magnetic copy can get one for the price of copying and mailing; please specify the word processing program. The glossary was prepared in Microsoft^® Word^TM, version 5.0, but can be provided in other formats, too: MS Word^TM for DOS; Word Perfect^TM 5.1 for DOS; MacWrite^TM; contact Dale if you're interested.

[2/28/2002: now posted as the 2nd rough edition/appearance in HTML format --and for others to freely download.]

We thank Fred Weitz, Paul Wing and John Dennis for recent Input.

If you would like to cite material from this glossary, feel free to do so; we place no restrictions on its re-use.  We are vain enough, though, that we wouldn't be offended by being cited. An acceptable format would be:

 Craig & Dale: A Stereo/Photo Glossary (2nd edition).  RWVCo, 1990.

Addresses:
   Craig: [6/23/2023] PO Box 861 / North Bend, OR 97459
   Dale: (sadly, deceased), Minneapolis, Minnesota

  Craig & Dale; May, 1993.

2/9/98: * Have now integrated items from "Stereoscopic Dictionary" appearing in the February '98 [Volume 5 #2 issue of Stereo Views, newsletter of the Cascade Stereoscopic Club.)

* This HTML document is now the only available format, as posted here.

8/6/2006: At this late date, Craig has finally learned (by happening across it) of an earlier coinage plus trademark usage of the term: "StereoSynthesis" by David M. Geshwind.

--Craig