APPENDIX A 315Most people can converge 32.5mm in 200, that is, look at an
object eight inches before the nose. This is a deviation of 16.25
prism diopters and a convergence of 32.50 prism diopters for both
eyes. On the contrary, most people find difficulty in fusing with
as little as 2.5 prism diopters divergence from the parallel.DEVIATION IN STEREOGRAPHY
Deviation is important in stereography, because of the varia-
tions in convergence necessary for fusing images of close-up objects
and also variation at a distance.
For example, when slides are to be projected, therewhould be
three prism diopters maximum variation between the nearest and
farthest images, and 2.5 prism diopter is even more advisable.
But it is important to note that this is a differential value, not an
absolute one. For example, assume that the camera lenses are
spaced 70mm, with a 35rnm half base. Then a deviation of one
prism diopter occurs at an object distance of 3% meters, although
to average vision (65mm) it would be 3% meters. A deviation of
three prism diopters would therefore be encountered at a distance
of 3.5/3=1.166+ meters.
Under the three prism diopter rule, objects which lie between
1.166 meters and infinity can be fused comfortably. If the 2.5 devia-
tion is used, the minimum distance will be 3.5/2.5 = 1.4 X 1.166 or
1.63 meters, about 5% feet.
However, if we compute the deviation for six diopters, we fincl
it to be 0.585 (58.3 centimeters) and 81 centimeters for the five
diopter distance (21Ax2).
Therefore, if we stereograph an object at a distance of 58.3~"
(23 inches in round figures), and have objects at infinity in the
same picture, the range of fusion is excessive and the stereogram
will not be comfortable to view. However, if the object at 58.3cm
is placed before a wall whose distance is 1.166 meters (46 inches
more or less), the range of deviation will be from +3 to +6, and
the stereogram will be just as comfortable to view as the one whose
range is from o to f3.
Using the 2.5 maximum the near distance would be 32 inches
with the background at 64 inches, giving a range from f2.5 to
+5 diopters.