Three-Dimensional Photography - Principles of Stereoscopy

APPENDIX A 317

SPECIFIC DEVIATION
If you will divide the focal length of the camera lens by 100,
you will have the nominal lateral value of one prism diopter
under taking conditions (variation in value caused by extension
of lens in focusing is ignored). Dividing the focal length of the
viewer lenses by 100 will give you the nominal value for the visual
prism diopter. The ratio of these two will give you a numerical
evaluation of the inherent distortion. For example, if the camera
has a prism diopter value of o.g.ijrnm, and the viewer has one of
0.50 diopter, the inherent stereo distortion amounts to 35:50 or
70: 100, which may be interpreted as a 30 percent distortion.
It is significant that only when the distortion exceeds 50 per-
cent (i.e., with the viewer twice the focal length of the camera),
or when the viewer has less than two-thirds the focal length of the
camera, does the distortion start to become visible.

PRACTICAL APPLICATIONS
A little experiment will soon show you the tolerance which you
have and the range through which you can deviate images with-
out detriment insofar as your specific audiences are concerned.
Once this knowledge is available, you can compute the character-
istics of your slides and mount them with accurate Compensation
for deviation. This produces an excellent slide for direct viewing
and for projection.