The Moving Picture World (April 1907)

THE MOVING PICTURE WORLD. 85 natural gift, as they followed their vocation of guess- work and accident, but to-day, photographic optics has been reduced to a series of mathematical equations, which must be satisfied in order to correct the various aberra- tions of sphericity and color, while the grinding, polishing and mounting of lenses is a trade which must be learned the same as any other. It is true that probably not one photographer out of a thousand has ever seen a lens manufactured, and it is true also that the great majority of optical establishments are not open to public inspection, so it is really no great wonder that many old-fashioned and erroneous notions regarding photographic lenses are still in existence. It was not so many years ago, that the diaphragm apertures in lenses were changed always to suit the shape of the picture to be taken; a square opening for a square picture, oblong for oblong pictures, and round ones only for round pictures. A scientific understanding of the relation of the apertures to field of illumination has done away with this absurdity. A large lens hood is also a relic of the past. There are a few who still insist, however, that a large brass ex- tension over the front of the lens will give more brilliant results because it shields the lens from external light re- flections. Fig. 1 It is easily demonstrated that the only gain of such an attachment is in weight. By referring to Diagram I, "L" represents the lens, "B" the lens barrel or tube, and "H" the hood. At a glance it will be seen that any light which can reach the plate through the lens and which is not intercepted by the lens barrel "B" Will not be affected by the large hood "H." s It is not so readily understood why the bubbles in the glass of the new Anastigmats do not leave their shadows on the plate. First, in order to demonstrate that they really do not, cut out a small disc of black paper about half the diam- eter of the lens, moisten and stick it on the front of the camera lens, leaving the diaphragm wide open. . If a photographer had been having imaginary troubles with bubbles in his lens, he would naturajjy expect to see the whole center of the plate darkened, but in reality the only difference, with or without the obstructing disc, is a difference in the strength of illumination over the entire plate. . A comparison between the area of the disc and the com- bined area of all the bubbles in the lens, in the worst cases, will make it evident that the slight difference in speed caused by this small loss of light is not enough to be noticed. Pig a. The optical demonstration of this is shown in Fig. 2. "L" represents the lens, "O" any point in any object to be photographed, and "I" is the image of the point "O" on the ground glass. "P" is the disc of paper. Observe now that all of the light proceeding from "O" which would strike the lens will be focused at "I", except that blocked out by "P", but as the obstructing disc only covers part of the lens surface, the other part which re- ceives the light will refract it to the image point "I". The point "I" was chosen as being located directly be- hind the paper disc and would be most likely to receive its shadow if it really cast any. The use of short focus lenses for general work is- the cause of many troubles. How many times have we heard such questions as these: "Why are my views all foreground and sky?" "Why does a small pond look like a full-grown laker" "Why are the hands and feet in all my portraits so larger "Why are the edges under-exposed in all my plates?" 'Why cannot I raise the front of my camera for high buildings without nipping the corners 7' "Why don't I get the relief effect so desirable in portraiture?" All these troubles arise from the use of a lens which is too short in focus. The first three are due to the inher- ent violent perspective caused by the difference in dis- tance from the lens to the center and to the edge of the plate. The reason for the under-exposed edges is that the image is magnified in the edge of the plate, but the illumination is not intensified by any means. In fact, the illumination is additionally reduced by reason of the diaphragm opening assuming an elliptical form when viewed from the edges of the plate.* The corners are nipped because the circle of illumination is not large enough, and the relief effect is lacking because short focus lenses have too much depth for portrait work and bring the background to about as sharp focus as the sit- ter. Authorities differ as to the proper length of focus *