280 THREE-DIMENSIONAL PHOTOGRAPHYdistance and is best determined by trial. For example, obtain a
pair of plane, 5 diopter prisms, Move the screen back and forth
until the images superimpose, Measure the screen-to-projector
distance. Also note the leeway that is the distance the screen may
be moved without separating the images more than two inches
right or left. Then try three diopter prisms and compare results.
This will enable you to determine the prism strength needed for
your projection conditions.
Practical Conversion.-Of all the methods recommended, the
compressor is best provided you can obtain the lens. (Original
cost around $80.) If the lens cannot be found, the next best is
the compromise masking which provides a square picture only
about^25 percent smaller than normal 16mm. The compromise is
of course by far the simplest of all methods and will probably
prove most attractive when all factors are considered.
Only as a curiosity do we mention the alternate system. This
provides full frame 16mm size, but it necessitates some kind of al-
ternating reflector, reciprocating or rotary, which could be made
only by an expert mechanic and would require connection to the
camera drive which adds to the motor load. Similar provision is
necessary for the projector. It is feasible but not entirely practical
for the average amateur. Film speed is 32 or more, so the camera
and projector mechanisms are subjected to excessive wear. Two
successive frames are required for one picture, so film cost is
doubled. It has been done and stereoscopically it is satisfactory,
but it requires special equipment as mentioned.
The compromise method does not permanently disable the cam-
era for planar work. If your camera has provision for aperture
masks, such as the Cine Kodak Special, the changeover can be
made in a few seconds so that stereo and planar can be alternated
in the same film. The only significant difference is that the camera
must be moved back somewhat from the subject. A compromise
film, in any modern projector will fill a 50x50 inch screen with a
good quality image.
Thus, while methods are used which do not include the re-
flector, those systems which are most practical are based upon
the use of the reflector.
Cost.-Reflector systems for the movie camera must be of pre-
cision quality. If you have the tools and are capable of fine