CLOSE-UP STEREOGRAPHY 243In the second method, assuming the camera has a lens separa-
tion of 70mm (half base 35), we may consider 20 inches as
equivalent to 50 centimeters, giving a deviation of seven diop-
ters. If we use one-half of this or 3.5 diopters, we place the back-
ground at one meter or twice the distance of the object. (40”).
In the third method, if the object lies at 50cm or 20 inches, the
nearest foreground should be not less than 43.75cm and the back-
ground not farther than 70cm. To substitute approximate equiva-
lents we have: Foreground 17G inches, principal object 20 inches,
background 27% inches. We have found such a distribution to
produce extreme close-ups which are as easy to view as the normal
long distance shot.
The vital point is not the absolute convergence, but the restric-
tion of deviation range within one slide to a value which is not
excessive for the scene of normal distances. A total range of three
diopters in normal slides equals infinity down to 1.16 meters, or
just under four feet.
But this can be misleading. The distance of 1.16 meters is the
distance equivalent of three prism diopters for a camera with
70mm lens separation, but for average human vision, three di-
opters indicates only about 42 inches. Therefore the range in-
volved in the close-up is that which would appear in a slide to
embrace distances from 45% inches to infinity, but which is
equivalent to direct vision form 42% inches to infinity (provided
the human intmpupillary is the average 65mm).
It can be seen from this that if you wish to make exact compu-
tations, you will have to consider both the separation of the lenses
of your camera and your own interpupillary separation. As a mat-
ter of fact the examples given are not precise because the camera
used actually has a lens separation of about 69+mm, but to make
computation easy, the round figure of 70 was substituted.
If you make any such computations you will be surprised at
the diversity of result, (as the three inch difference in the three
diopter limit between camera and eyes). In actual experiment you
will be equally surprised to learn the great divergence from theo-
retical limits which can be made without the introduction of any
visual distortion. In fact most stereographers use the 65mm base
as normal for all computations regardless of the camera used. The
only exception is that made for extended or diminished base as in