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1953-54 Theatre Catalog, 11th Edition, Page 255 (219)

1953-54 Theatre Catalog, 11th Edition
1953-54 Theatre Catalog
1953-54 Theatre Catalog, 11th Edition, Page 255
Page 255

1953-54 Theatre Catalog, 11th Edition, Page 255

iems as the conventional still photographer, plus a few extra problems that are peculiar to stereo. Interocular distances, interaxial distances and convergence problems are some of the extras which lie only in the province of stereo. But they are soon mastered and the pleasure derived from their mastery is rewarding indeed.

A difference in focal length between the lenses of a stereo camera of more than one-half of one per cent will produce images differing enough in size to cause eyestrain when pictures are projected.

If glass filters are used on the lenses, there may be enough "wedging'l in the filter assembly to throw the picture axes out of line. This is not serious if the misalignment is along the horizontal, but it may be significant if it is along the vertical, and when the pictures are projected, it may be found that they have disturbing vertical displacement between them. This ffvertical" may not produce a vivid consciousness of eyestrain, but it may very well produce enough effect to tire the observer after a time. As J. M. Dalzell has said, ftAny sensation of visual lassitude, discomfort or even slight pain, is a psychological and mental affair. It is the penalty for exceeding certain geometric limitations which Nature has imposed upon the use of two eyes simultaneously. Its cause is cumulative and lies in the breaking* of one or more of a few exceedingly simple rules*"

To avoid eyestrain, all elements in every plane must have crisp definition such as good eyes provide in their wanderings from plane to plane when looking at a scene. Out-of-focus or ttsoft focus" areas, whether in the foreground or background, are an abomination that prevents visual comfort in viewing the stereogram.

Since the normal two and one-half inch lens interocular ordinarily should be used on objects no closer than about ten feet, it is apparent that a properly made stereogram can be taken only in a camera having a variable interaxial. Of course, some shots may benefit from a larger interaxial than normal, particularly in cases where an increased depth ls considered a dramatic embellishment. By increasing the interaxial several times normal, the depth of such stereograms can be startlingly exaggerated.

The variable convergence feature is a Hmust" on any versatile stereoscopic camera because it permits the arrangement of each picture within the desired area. To be able to converge to any plane, preferably in front of the nearest object, eliminates the marginal disturbances that occur when the lens axes are parallel. Such marginal disturbances are particularly annoying when they exist In the projected stereogram. The ideal stereogram should be a ureal-life," view as seen through a window. Therefore, no Object should be permitted to invade the plane where the window exists if that Object "touches" any part of the window frame. If it does, the result is a distracting border which gives non-coincidental image reconstitution.

SEEN ON THE LEFT: The side margin effect with nonconvolqonl lanul. RIGHT: The effect with lenses converging on a foreground plane.


If the ftwindow" is at infinity, as it will be with parallel lens axes, the illusion will be that foreground middle distance and part of the far distance planes are between the window and the observer, a totally abnormal effect.

To create a proper window illusion the masks framing the images must be identically alike in shape and size and their horizontal margins must be level.

Masks can be of any shape. For instance a key-hole mask will provide a stereoscopic View through a key-hole, an anomalous effect since key-holes are so small that real-life peering through one is limited to one eye.

Where objects do not touch the window frame (the picture margins), they can be permitted to come through the window and even made to seem suspended in space between the screen and the observer. In table top photography, one can produce this effect by mounting the subject on glass, being sure it is well inside the picture area and not touching its margins at any point. Convergence is made to occur behind the subject. Viewing comfort is then readily achieved.

Convergence is best accomplished by moving the lenses, not by tttoeing-in" the camera bodies. Toe-in introduces a "keystone" distortion that makes the right edge of the left eye picture shorter than the left edge, and the left edge of the right eye picture shorter than its right edge. This kind of distortion may not be considered important, and will be tolerated by some people, but it is one of the things to be avoided if the stereo worker wants to insure that his projected stereograms provide complete visual comfort.

To make a satisfactory stereoscopic picture, the camera must be level and to project the images successfully, there must be no vertical displacement between them and they should not ftrotatefi

LEFT EYE r'/r*--r I I . as. . I In \\ 6i

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one with the other. In other words, the angular alignment has to be the same for both members of the pair.

We may work our interaxial problem out by "rule of thumb." Stereograms that are perfectly acceptable, if properly framed in projection, can be made by taking pictures only of objects further than 50 times the interaxial employed. (This is for camera lenses of the medium angle usually employed.) This rule limits the nearest object to a distance of ten feet with an interaxial of two and onehalf inches. It is violated to an extreme if two and one-half inches is used for a five foot distant object, where the indicated interaxial is one and one-fourth inches.


Bringing out a modern camera for three-dimensional use was not enough to stimulate the rebirth of interest in the stereoscopic art. There had to be some device enabling people to see the threedimensional picture. The manufacturer of the Stereo-Realist appreciated this from the outset, and accordingly brought out an individual vieWer. Similar cameraviewer combinations have appeared since the Realist made its debut. But an individual viewing device is hampered by an obvious shortcoming; it limits the please me of looking at a three-dimensional picture to one person at a time. This shortcoming does not apply, however, to projected stereo pictures. Shortly after the introduction of camera-viewer combinations, a three-dimensional projector became available and it enables many people at the same time to see and enjoy the same stereo pictures. There are now several stereoscopic slide projectors on the market, all more or less alike.

In addition to the added enjoyment it provides, the projection of'stereograms

1953-54 Theatre Catalog, 11th Edition, Page 255