CHAPTER 9PICTORIAL STEREOGRAPHYNY STEREOGRAPHIC EXPOSURE must include such technical con-
siderations as exposure, filters and aperture, and it must
also include certain fundamental esthetic considerations which
may roughly be divided into two groups. The first group includes
those factors which, while not strictly photographic, have much
to do with the stereo perception, including the type of lighting to
be favored. The second group includes the elusive considerations
of pictorial composition as applied specifically to stereography. As
no stereographic reproduction can be made without giving more
or less consideration to these factors, we shall discuss them to-
gether.
The whole subject of three-dimensional photography has often
been discussed so irrationally that we must pause for some clarifi-
cation. In the first place, three-dimensional photography is not
necessarily identical with stereoscopic photography; and even
more decidedly, binocular vision (or photography) must not be
regarded as identical with stereoscopic vision (or photography).
Any photograph which exhibits a definite appearance of depth
may correctly be called three-dimensional. Any good photograph
examined in a concave mirror exhibits a three-dimensional qual-
ity, but it is not stereoscopic.
PSYCHOLOGICAL DEPTH PERcEPTIoN.-It is evident that we must
consider the true nature of visual depth perception before we can
arrive at any conclusion about the nature of stereoscopic vision.
The several factors which combine to provide the visual percep-
tion of depth or distance are:
I. Geometric Perspective.-This is inherent in every planar
photograph and in good drawings (ancient and oriental art ex-
cluded). It is the arrangement upon a single plane of those aspects
of an object which in reality occupy three dimensions. For ex-
ample, a side of a cube is not shown as a square but as an irregu-
larly quadrangular shape with the far side much shorter than the
near one.
In actual fact perspective relates to the size-in-width element of
the depth factor, and is always limited to those planes which are
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