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140 HANDBOOK OF PROJECTION FOR
measured with a micrometer caliper. This is especially important if one projector be new and the other quite old.
RANGE OF FOCAL LENGTH.— Projection lenses from 2 to 8-inch E. F. are carried in stock by manufacturers.
MATCHING LENSES.— Do not attempt to order a lens to match one you have by giving the focal length on the lens barrel. These focal length markings cannot be depended upon for close work. They are usually only approximate.
If you have a lens and want one to match it you must send the lens to the manufacturer to be measured, or else give the manufacturer the precise width of the projector aperture as measured by a micrometer caliper, the exact distance from aperture to screen and the precise width of the light upon the screen when there is no film in the projector.
Even with these precautions you cannot be certain, because an error involving the smallest fraction of an inch may result in unsatisfactory results, if the error be made at the right place. If there is a keystone, the distance from screen to aperture must be measured at the same distance from top or bottom of picture that the width of picture is measured. Better send the lens. It is the only sure way.
When lenses are purchased in pairs they are usually matched by selecting lenses from stock. Suppose two lenses are wanted to project a picture of a certain width at a certain distance. The dealer or lens maker finds by computation that this requires a 5-inch E. F. lens. He tries one and finds it gives a picture a bit too large. He tries another and another until one is just right. He then, by a process of selection through tests, finds another which gives exactly the same size picture at that distance.
Why is this, you ask? Why did not the 5-inch lens give what it was supposed to give? For the reason that lenses vary, and their markings are not accurate. A 4-inch E. F. projection lens may vary from 3.95 to 4.20 inches, according to the admission of one large lens concern. Exhibitors and projectionists should remember this when matched lenses are wanted.
NOTICE. — The Gundlach Manhattan Optical Company marks the exact actual focal length of its projection lenses in the invoice sent to purchasers. This notation is in figures and in parenthesis.
This bit of information should be carefully filed away in the projection room, because if a lens of that focal length is ordered later it will match the one you have.