CLOSE-UP STEREOGRAPHY 245which varies with the stereo skill of the individual. When the
object distance is small, 16 inches or less, and when it has con-
siderable depth, there will exist a condition of excessive difference
in parallax as the eyes move from one part of the object to an-
other. This is known in stereo work as an excessive depth of
parallax, to some extent analogous to depth of field.
When there is an excessive parallax depth, the spectator is
conscious of the change in convergence in looking from the near-
est to the farthest point of the object. Some, particularly those
ivho are new to stereo, find this actually painful. Those who have
had more experience in stereo viewing and have developed some
stereo skill do not find it uncomfortable, but the sensation re-
mains, including that of a definite time interval in moving from
one position to the other. Whether this is objectionable or not
and the degree to which it is objectionable, depends upon the
individual. As a rule, however, it is advisable not to make full
base stereograms at a distance of less than 12 to 14 inches, even
when prisms are used to eliminate the ghost side-images. For
nearer distances, macro techniques are advisable.
MAcRo.-The stereo camera is easily adaptable to macro photog-
raphy, and with suitable supplementaries consisting of prisms
with positive correction ground in, it is possible to use the normal
stereo camera at distances as close as ten inches. This is approxi-
mately 25omm, and as the usual focal length of the camera is
35mm, the ratio will be approximately i:7. However it must be
remembered that the parallax remains normal. That is, if the
object is at eight inches, the parallax will be that of an object ac-
tually viewed at eight inches; and, as explained, the relief will ap-
pear to be exaggerated and parallax depth will be excessive.
Many workers object to relief which results from any parallax
greater than six or eight feet. Personally the writer does not agree.
He finds the parallax of 16 to 20 inches quite acceptable, and for
many objects even preferable to a compressed relief. It is advis-
able to limit this technique to a minimum of IP inches.
Parallax increases as distance decreases. “Greater parallax” re-
fers to nearer objects.
SUPERMACRO.-FOT that type of macro shot which merges into
low power micro, it is ordinarily necessary to use two cameras.
Zeiss makes a stereo ophthalmic camera for making macro-stereos