Three-Dimensional Photography - Principles of Stereoscopy

186 THREE-DIMENSIONAL PHOTOGRAPHY
closeup objects it is spurious-if it is visible throughout the scene,
in correct proportion, then it is real. It is curious that most of
those who complain of excessive foreground relief complain of
deficient background relief at the same time. This is an impossible
condition.
Most of the failures of projection are due solely to the spectators
being unwilling to acquire that visual skill which will cause the
elimination of these bugaboo, makebelieve distortions,
PUBLIC PRO JECTI0N.-This poses a real problem because here we
have spectators who are usually inexperienced. Also because of the
numbers, it is impossible for more than a few to be seated within
even the approximately correct positions. What can be done for
them?
It is fair to assume that if the exhibition is public, it may be
assumed to be commercial. Then a slight investment is not objec-
tionable. The easiest way to overcome the defect, and one which
will offer other advantages to the stereographer in making pic-
tures, is to have the camera equipped with long focus lenses. Thus
if the focus is increased to three inches, the preferred spectator
distance jumps back to about 15 feet, and if these lenses are around
five to six inches in focal length the distance drops back to around
30-35 feet.
The narrow angle also provides against perspective distortion
which exists in looking at the conventional image from a greater
distance. Of course the screen size is small, due to the spectator
distance, but inasmuch as these distances are fixed in the average
auditorium anyway, there is no decrease in screen size over that
which would be seen with the distorted depth. It is this very de-
crease in normal screen size which we are trying to compensate.
However with three to four inch lenses and a loo inch screen we
also move the distance back to around 30 feet more or less, and
then we have a big picture, a convenient angle of vision and a
depth factor which will easily satisfy spectators between the dis-
tances of 15 to 60 feet.
There are other ways of reducing parallax, but this is the most
convenient inasmuch as it is not too difficult for a good camera
mechanic to install such lenses. It might even be practical to make
the lens boards interchangeable, but so far the writer has found it
more convenient to mount the two lenses upon a separate camera.