STEREO MOVIES 279area gain of almost i/3 over the 8mm conversions, and provides a
highly satisfactory format. For rapid comparison of effective aper-
ture areas we have:
8mm conversions, 3.75~5 = 18.75 sq. mm.
Compromise, 5.5~5 = 27.5 sq. mm.
16mm 7.5~5 = 37.5 sq. mm.
The area ratios are roughly 2-3-4.
For those who do not object to the slightly smaller picture, the
compromise is the best format to use. It differs from the straight
8mm frame in being slightly higher than its width, while the nor-
mal 8mm is wider than high.
In all of these conversions the width of the frame is that of the
normal 8mm frame. This can be increased ONLY by (A) using a
35mm camera to obtain masked 16mm screen size, (B) using a
compressor to provide an effective frame width of about 7.5mm,
or by using two separate cameras.
When using the compromise system, it is not necessary to mask
the projector. When the camera aperture is masked, the unex-
posed film will be blackened in processing thus making an auto-
matic mask for the film. This method together with simple prisms
to deflect the projected images is the simplest of all stereo movie
methods.
The two prisms are placed base to base and then placed over
the projection lens so that the junction of the prism bases lies
vertically. The strength of the prisms depends upon the screen
Fig. 19-4. A projector prism Fig. 19-5. A simple projector
which has been cut to circular prism made by placing the thick
shape. edges of two prisms together.