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244 AMATEUR MOVIE MAKING
perfect exhibition, yet there are no makeshifts, no furniture wrestling. One small case contains the whole outfit which is set up in a few moments, and as quickly packed when the evening's entertainment is over.
Before purchasing a screen it is necessary to determine the focal length of the lens to be used with the projector. This is a subject which proves confusing to most people. The projection lens must be placed a definite distance in front of the film. This distance is determined by the focal length of the lens used. A short focus lens is placed nearer the film than would be one of long focus. It follows then that the shorter focal length lens, being placed closer to the film will emit a wider angle of light than would a lens farther from the film.
The shorter the focal length of the lens the larger will be the size of the projected image, all other things being the same.
It also follows that, inasmuch as the light is projected from the lens in the form of a cone, the farther the screen is from the projector, the larger will be the image. The question is, "How can the proper focal length be determined."
In all sixteen millimeter projection we have one fixed factor, the size of the individual frame. The screen size, throw (or distance between projector and screen) and focal length of the lens used are all variable. The screen size is determined by the preference of the owner, and by the intensity of the light used. The throw is largely determined by the size of the room in which the projection takes place and the focal length of the lens is determined in turn by these two factors. For example suppose that we have a distance of ten feet between projector and screen and we want to secure a picture about three by four feet, or slightly less for projection upon a screen of that size. By referring to the table in the Appendix we see that a lens of one inch focus will give us a picture 2.85 feet by 3.85 feet giving us something near a one inch margin all around the image when projected upon the three by four foot screen.
As the 30 x 40 inch screen has become almost the