ELEMENTARY STEREOGRAPHY 11panchromatic, but because of the antagonistic factors we have
already discussed, a moderately high sensitivity is often very valu-
able in stereography. The popular panchromatic emulsions rated
at approximately ASA^50 are perhaps the best all-around stereo
emulsions. It is to be understood that like all generalizations, this
is subject to variation for any of the many special types of sub-
jects constantly encountered in stereoscopic work.
Of course the ideal material is color film. There is much to
be said in favor of monochrome planar pictures, nothing in
favor of monochrome stereo because color is an active stereo
factor aside from its esthetic weight.
When the 35mm camera is used, the shorter focal length makes
it possible to use larger apertures. Therefore the slower speed of
color film loses its significance, and in such miniature cameras
stereography with color film becomes as easy as, or in fact easier
than, black and white with a larger camera.
FACTORS OF DEPTH PERCEPTION.-There are several factors
involved in our sense of visual depth perception, all of which
can be used to good advantage in stereography, but as these be-
long to the division of advanced work, we shall leave a discussion
of them for a later chapter.