Three-Dimensional Photography - Principles of Stereoscopy

178 THREE-DIMENSIONAL PHOTOGRAPHY
do, we need not be concerned with such variations because the dis-
tortions introduced by variations of this size are not visible as
such. In short we have a plus-or-minus leeway of at least ten per-
cent, so the task is not at all difficult.
The camera, let’s take the Realist as an example, has lenses
whose focal length is 35mm. The picture as projected will have a
width in the mount of say 22mm. Therefore your field angle is
that of a triangle whose base is 22 and whose height is 35. You can
make that inches or feet or meters and you will not change the
essential angle.
Now all that is necessary is to place the spectator at a distance
which shall be 35/22 of the screen width. If the picture is 50
inches wide, 1/22 of that is 2.27 inches, lets say 1% inches. 35 times
that is 78% inches or about 6.5 feet. But of course the Realist has
about a 10 percent greater interocular than normal, so to get a
rough estimate let’s just add ten percent to our distance and we
get 86.62 inches or just over seven feet. This does not show the
exact mathematics, but we have no need for precision results and
the above is the kind of computation which can be carried on men-
tally without paper or calculating machine.
Note that the focal length of the projection lens does not enter
into the thing, the only significant factors are: film width, that is
of the free opening in the mount; screen picture width; field angle
of camera for the free film width and spectator distance. In short
your problem is simply to establish an included angle of the field
of vision which duplicates the camera field angle. That is all, as
far as basic conditions go, but there are details to be considered.
It is assumed that your slides are in alignment as has already
been described. A slide which shows a vertical misalignment
greater than that which appears as a slight doubling of the image
should be re-mounted. Do not depend upon the projector adjust-
ment to align poorZy mounted slides. This is the most frequent
source of trouble in stereo projection as such adjustments, made
when the spectators are viewing the screen, is almost sure to result
in making at least some of the spectators actively ill!
When the slide is projected, there are two images visible upon
the screen. Because polarized extinction as applied to stereo pro-
jection is no longer a novelty, I shall not go into any detailed ex-
planation of this phenomenon. It is enough to say that when you