Three-Dimensional Photography - Principles of Stereoscopy

1 52 THREE-DIMENSIONAL PHOTOGRAPHY
The perception of stereo relief is that sensation which enables us
to see (not judge or interpret) the most minute details of differ-
ences of depth. It is something impossible to describe to those who
have not experienced it, something which can with difficulty be
distinguished from the extrinsic evaluations when it is described
to those who have stereo vision, but which is instantly and obvi-
ously apparent as soon as a stereogram is viewed.
The difference is overwhelmingly great, yet it is subtle. For that
reason, even when one is aware of the problem involved and
makes every effort to determine the boundary between stereo and
extrinsic depth evaluation, the limit can be only approximately
defined.
Because the stereogram reproduces all of the effects of the ex-
trinsic factors as well as those of the intrinsic parallactic factor,
the stereogram shows you what the eye would have seen. True,
those who have studied the subject for years can see the flatness of
distance either under direct vision or when viewing the stereo-
gram.
But after all, this is merely the splitting of technical hairs. You
are not interested in the psychology or the physics or the mathe-
matics of stereo. You want to make attractive stereograms. That is
no doubt the full extent of your goal, even as it represents the
ideal ultimate of stereoscopic photography.
Then, the one thing you want to know is this: The limitations
of the normal stereo camera are the limitations of a normal pair of
eyes, because “What you see, you get.”
At the same time, objects relatively near, say at distances of
three to 20 feet, are the most clearly delineated to vision because
truly familiar objects are usually seen at such distances. You will
find that stereograms made within these limits will usually afford
you the greatest degree of satisfaction. This is not true because
of any limitations imposed, but because it represents that visual
zone which is the most satisfactory in direct vision; and any ex-
ceptions which apply to vision apply to stereo.
It cannot be denied that there are people who have trouble in
seeing stereograms under any conditions, and this trouble can be
traced to the fact that these people still have accommodation and
convergence coupled. It is fundamental that there is no type of
stereogram which can be seen without splitting these two func-