CHAPTER 12STEREO PROJECTIONHE NEAREST APPROACH to perfect stereo reproduction is that
T achieved when a hand stereoscope is used, one which matches
the camera in focal length. However this is a rare condition inas-
much as practically all stereoscopes have lenses of greater focal
length than those used in the camera. As thousands of stereo ama-
teurs will testify, the space distortion although it exists, is not ap-
preciable in such stereoscopes.
Naturally, as the stereoscope is limited to a single spectator, it
can hardly be classed as a social device; and as most of us like to
share our enjoyment, stereo projection has proved potentially
more popular than the stereoscope. It is unfortunate that the gen-
eral acceptance of projection has been marred by certain condi-
tions that are the fault of the stereographer rather than of projec-
tion itself. So great has been the confusion that each stereographer
has his pet theory of stereo projection, most of them incredible.
They all seem to be based upon “one-shot” experience and display
such an airy disregard of even the most elementary stereo theory
that one wonders how the pictures could be viewed.
Too many people take optical projection for granted and never
think about the principles involved, and when the projection hap-
pens to be stereo with its added principles it is extremely easy to
get badly tangled up. Let’s try to unravel the tangle.
Parastereo Projection.-The ideal of orthostereo projection is a
physical impossibility because one of the essential factors is that
the spectator’s eyes shall be directly in line with the axes of the
projection lenses. Obviously the head would then cast a shadow
over the screen, blanking out the picture. The next best thing is
to establish the conditions of orthoprojection and then move the
spectator the minimum distance which will permit the picture to
be projected upon the screen. Because this is a near approach to
orthoprojection we shall call it paraprojection even though it does
not correspond in type to parastereoscopy.
The first step is that of making the stereogram. This records the
details of a certain scene before the camera. The ideal of projec-
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