242 THREE-DIMENSIONAL PHOTOGRAPHY
same slide, but there are many who regard such an appearance as
a “distortion.” It is nothing of the kind, simply a phenomenon
which is part and parcel of the differential homologous distance
which in turn is the essential factor of stereo.
Thus, the decision sinks to the level of personal preference, but
in stereo there is a habit, and a bad one, of condemning every-
thing which violates personal preference, as a “distortion.” There
can be no true laws or rules which do not apply equally to all
individuals concerned. Thus the establishment of a minimum
working distance is akin to the problem of the window. Many
workers insist that the window MUST be nearer the observer
than any part of the subject; others find that having the window
in midplane actually exaggerates the appearance of relief, so there
again, as we have said, the matter is not one of rule, but one of
personal preference and nothing else.
However, if you prefer to avoid the characteristic appearance of
the close-up object, you can do so easily, but it will be necessary
to keep the background within a distance which does not exceed
the object distance by more than five to ten times. Using one
camera, you may center the object for both exposures by rotating
the camera and moving it laterally between shots. There is no
limit to the object distance in this case other than that imposed
by the working capacity of the camera used. If you use a stereo
camera, you may place prisms over the lenses which deviate the
optical axes so that they intersect at or near the object.
The background limit of five to ten times the object distance is
maximum and should not be used if the very best results are
desired. Actually the deviation of the principal object should be
determined, and the background placed at a corresponding dis-
tance. According to one school the farthest background should
have a deviation of not less than half that of the principal object,
but we prefer to keep the background within three prism diopters
of the nearest foreground. This will permit the principal object
to be placed, for example, one diopter behind the immediate fore-
ground and two diopters in front of the background.
The difference can be illustrated by examples. Assume the ob-
ject distance to be 20 inches. According to the first method (five
to ten times object distance), this would place the maximum back-
ground at 100 to 200 inches, roughly 8% to 17 feet.