STEREO PROJECTION 177farther one is visually only half as high and half as wide as the
nearer.
Visual size is a matter of included angle rather than of feet and
inches or other linear measurement.
Suppose the near end of the log was seven feet from the camera.
You take your position seven feet from the screen, and you will
see the log extending back of the screen a distance of six feet. Now
move back until you are 14 feet from the screen.
Although the screen size is now only half what it was you are not
conscious of this size change. Eut, the length of the log, the fixed
factor remains the same. There is a great discrepancy in the depth
dimension and the width dimension, so instead of making the cor-
rect judgment, we base the visual effect upon comparative or rela-
tive sizes and we say at once that the log is now 12 feet long.
And there is still another factor which adds to this error. At a
distance of 14 feet the log still shows the parallax of seven feet, and
if this were to happen in real life it would be found that the log
is 12 feet long because in real life a log which shows the same par-
allax at 14 feet that a six foot log does at seven feet, must be 12
feet long. But the simple fact remains that the cause of size distor-
tion, specifically depth distortion lies, not in any defect of projec-
tion technique, but rather in the strict operation of optical law.
The most often voiced objection to projected stereo is based
upon this effect of disproportionate size-in-depth and size-in-width.
Nor can this effect be remedied fully until there is developed some
means of providing variable parallax so that the parallax may be
altered by each spectator to correspond to his individual visual
screen angle. So far no discovery has been made which affords even
the barest hint as to how this fundamental can be altered. It is one
of those things which almost justify the description “impossible.”
So there is the obvious conclusion. There is only one distance
from which a screen image of specific size can be viewed with
theoretically accurate results. That is a fact but there is a bit of
silver lining.
Paraprojection Factors.-How are you going to determine the
factors of parastereoscopy? Simply as in orthostereo, and as has
been hinted in the paragraphs immediately preceding. Your aim
is to keep the visual angle the same as that of the camera. Note that
although cameras vary in their interocular separations as humans