4 THREE-DIMENSIONAL PHOTOGRAPHY
the present we shall ignore them and proceed to make the simple
stereogram.
THE SIMPLE CAMERA.-A stereoscopic camera or a stereoscopic
adapter for a standard camera serves but one purpose, that of
making the two exposures simultaneously. This permits stereo-
grams to be made of moving objects, but except for that purpose,
any camera, including the simplest box type, may successfully be
used for stereography. If you have a camera which you ordinarily
use, you can make stereograms of motionless objects with it.
THE POINTS OF VIEW.-Theoretically, the two exposures should
be made with the optical axes parallel and with a distance of
65mm between the points of view. We shall see that even in the
best of technical stereography these factors are often ignored, bu?
for the present we shall accept them as a working standard. In
this connection it is interesting to note that the interpupillary
distance varies with individuals, and that while 65mm has long
been the accepted standard in the metric system (with both 6omm
and 65mm as occasional competitors), 2% inches is the accepted
standard in U. S. measurement. Therefore, the two exposures
will be made by moving the camera approximately 2% inches
between exposures.
CONTINUITY OF BAsE.-The open frame in the rear of the cam-
era against which the film lies and which frames the negative, has
a position relative to the lens axis which is mechanically fixed.
This means that the optical center of the image is always the same
distance above this frame baseline.
It is essential that in the completed stereogram the common
base of the two positives must be parallel to this camera base, and
that the two exposures must be made under the same conditions.
Thus if the two exposures are made freehand, as is not un-
commonly done by an expert stereographer, one of them with a
slight tilt to the right and the other with a similar tilt to the left,
there is no original continuity of base, and the prints must be
aligned by stereoscopic analysis, a task for the experienced stere-
ographer. Therefore, while freehand exposures are often made
successfully by the experienced stereographer, the beginner will
almost inevitably be disappointed if he tries it.
THE STEREO SHIFT-HEAD.-TO avoid the condition just de-
scribed, some kind of stereo tripod adapter or shift-head should