CORRELATION OF CAMERA AND VIEWER 61gain resulting from using long-focus camera lenses is exactly neu-
tralized by the loss resulting from the long-focus viewer lenses.
Thus no matter what the focal length used, the results are identi-
cal, with the sole exception that the shorter the focal length the
greater the area embraced within the field. Otherwise every object
has that size-distance character possessed by the original as viewed
from the camera position..
Inasmuch as both stereomicrography and telestereography are
of importance, it is highly desirable that some such techniques
may be used. Various methods have been suggested in the past,
but only parastereoscopy fully meets the demand for telestereogra-
phy. Parastereoscopy is substantially the same as orthostereoscopy
with the exception of one condition whose importance is far
greater as a matter of theory than as a matter of practical appli-
cation. In fact, parastereograms have no distortion which is dis-
cernible to vision. Measurement alone will reveal the difference.
Parastereoscopy is made by strictly observing the PePax relation-
ship, which is done by altering the stereo base and focal length
in exact ratio.
If the camera and viewer are normally equipped with three-
inch lenses, then that is the normal for that particular pair of
instruments. If it is desired to use six-inch lenses, then the base
is doubled by moving the lenses to 130mm separation. The re-
sulting stereogram will be viewed without perceptible distortion
in the normal viewer.
The use of long bases is difficult with the ordinary camera,
particularly when simultaneous exposures are desired. My first
PePax reflector was made upon the sliding tube principle, and
later I substituted one designed along the lines of the periscopic
stereobinocular, with barrels rotating in an arc to provide the
desired separation. This, however, was a purely experimental
device and has not been commercially manufactured. These re-
flectors are in every way similar to the conventional stereo reflector
with the single exception that the internal reflectors instead of
being positioned immediately adjacent, are separated by the nor-
mal stereo base. This construction makes the reflector applicable
to the normal stereo camera and provides any desired base up to
the maximum, which may reasonably be ten times normal, or
65 centimeters.