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LENSES AXD MIRRORS
Why and Wherefore of Focusing
121
(91) From every pinpoint of the film photograph
posed over the projector aperture, millions of diverging light rays go forward in the form of a cone, the taper of which is dependent upon the focal length of the condenser or reflecting mirror and its distance from the aperture. (92) It is the function of the lens to collect these rays from every pinpoint of the photograph, and to refract them so that they will converge and focu at similarly placed but enlarged points upon the screen. The film represents one conjugate foci point of the projection lens, the screen the other. (93) If the distance of the lens from the film is altered in any degree by means of the focusing screw, or by other means, the definition or focus of the screen image is changed.
Figure 38
(94) In Fig. 38, we find an explanation. A, being the film, B the projection lens and 1 the screen, it is observed that at 1, the point of focus, the rays emanating from the film photograph meet the cross. If the screen were at 2 instead — and not at 1 — the rays would not focus at 2 unless the conjugate foci points were altered. On this plane each point would appear as a circle and the whole image would be badly blurred. When the screen image is blurred it indicates that the rays are focused at a point or plane either before or behind the screen surface. The lens must then be moved slightly backward or forward, altering the conjugate foci points so that the rays focus exactly at the screen surface of the plane.
(95) Depth of focus is definitely a physical condition.