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MANAGERS AND PROJECTIONISTS 155
ture of given size (width) at a given distance, proceed as follows :
Measure the width of aperture of your projector accurately (if it is a stereopticon lens, then the standard slide mat width, 3 inches, is used) by means of an inside caliper, though if the projector be of late model we may take the aperture width at .90625 (29/32) of an inch with assurance of pretty close accuracy. Next measure the exact distance from the center of the lens barrel to the screen. Multiply the distance from the lens to the screen, in feet, by the width of the aperture, in fractions of an inch, and divide the result by the width of the picture you desire, in feet.
The result will be the E. F. of the lens required to project a picture that width. It will be as close to it as you or any one else can get by figuring. If the lens itself is accurate as to E. F., the result also will be accurate.
For instance : Suppose we desire a fifteen foot picture at sixty feet. The projector aperture is found to be .90625 (29/32) of an inch wide — the new standard. We first multiply the distance from the screen, in feet, by the width of the aperture, .90625, which gives us 54.3750. Dividing this by the width of the desired picture in feet we get 3.625. We would therefore require a lens of 3.625 inches E. F. to project a picture exactly 15 feet wide at exactly 60 feet.
It would probably be impossible to find a lens marked exactly this focal length. The most practical method is to determine the width of the picture you want and the exact distance from the lens to the screen, supplying this data to the lens dealer. He will probably be able to select a lens from his stock which will meet your requirement.
The stereopticon lens is figured in exactly the same way, except that instead of measuring the aperture width we accept 3 inches as the standard slide mat width — the slide mat being, in this case, the aperture.
For the benefit of our readers we append the formula by means of which certain other factors may be figured:
The size of the image which a lens of given E. F. will project at a given distance may be found by multiplying the difference between the distance lens to screen and the focal length of the projection lens (E. F.) by the width of the aperture, and dividing the product by the E F. of the lens. For example, let L equal the projection distance, 40 feet (480 inches); S the slide mat (3 inches), and F the E. F. of the lens, which we will assume to be 12 inches. We then