316 THREE-DIMENSIONAL PHOTOGRAPHY
The adverse criticism leveled against the very close close-up in
stereo may be justified in those instances where there is a distant
background, but the criticism is not warranted when the total in-
cluded range lies within limits. Most of the criticism against the
close-up originates in the fact that many stereo technicians have
inherited a tradition but are unfamiliar with its technical nature.
A close-up is, to them, inherently poor regardless of any and all
other circumstances.
DEVIATION IN PROJECTION
JYhen projecting a series of slides, it is advisable to have them
pre-adjusted, because if the lateral separation is set for distance
subjects, the projector will maintain the infinity separation and
close-up subjects will require compensating adjustment.
Slides €or projection should be arranged so that the separation
of the furthest images should be uniform. For example, in the
usual 35mm slide, assuming it has been made with lenses of 35mm
focal length, we may assume one prism diopter to have a lateral
value of 35/ioomm, which is roughly one-third millimeter. Let
us also assume that in standard mounting, the infinity-homologous
distance is 65mm. Slides which include infinity, have images of
objects at infinity separated by 65mm. But such slides should have
no images separated by less than the limiting deviation of three
prism diopters or a total of six prism diopters for the two units.
Therefore the images of the nearest objects should be not less than
65 - 2.1 or 62.gmm apart.
If, however, the widest separation (image of most distant ob-
ject) is 62.9, then the nearest may be as little as 60.8 in separa-
tion. In that instance, the films themselves are moved apart
2.1mm. This slide which represents an original field with a depth
extending from 23 to 46 inches, will project just like the one
which represents an original range of 46 inches to infinity.
The whole technique lies in keeping the images of the most
distant object at the nominal homologous separation for infinity.
This applies to slides made at a range of 1 1% to 23 inches, as well
as to the other two examples cited.