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122 RICHARDSON'S BLUEBOOK OF PROJECTION
(96) It is in exact proportion to the speed (diameter) of a lens and its focal length. (97) All lenses having equal quality of correction, equal focus and equal diameter will have exactly the same depth of focus.
Altering Lens Focal Length
(98) It is impossible to alter the E. F. of a lens by lengthening or shortening the distance between its front and back factors — by lengthening or shortening the tube in which they are mounted — without, at the same time, destroying its maximum efficiency. It is a bad practice.
Picture Size and E. F. of Lens
(99) Any lens will project a picture any practicable distance, but (100) a lens can project a picture of given dimensions at one, fixed distance only. If it projects further, the picture dimensions will be increased and vice versa. Any alteration of projection distance automatically alters the working distance of the lens. ( 101 ) The greater the E. F. of the lens the longer will be its working distance — distance of the surface of the rear lens to the aperture. Moving one conjugate foci point further from the lens automatically moves the other closer to the lens.
Measuring Focal Length
(102) Due to spherical aberration and other faults of simple lenses, the projectionist cannot measure their focal length accurately unless he uses a gauge for measuring lenses in diopters. With this instrument, focal length in inches is easily calculated. (103) He may, however, measure their focal length for all practical purposes by another means — crude but sufficiently accurate.
(104) To measure the focal length of a piano convex lens proceed as follows: Select an open window from which some object not less than fifty feet nor more than one hundred feet distant is visible. To the wall opposite the window pin a sheet of white paper. Hold the lens with its flat side toward the screen and parallel with its