International photographer (Jan-Dec 1935)

Twenty The INTERNATIONAL PHOTOGRAPHER April, 1935 THE SPEED OF LENSES By F. M. Steadman (Being comments on the November and December articles on that subject by Messrs. Melson and Lawrence of Bausch isf Lomb in The International Photographer.) ELSON and Lawrence explain correctly that the volume of light is a factor of lens speed and that this volume is "regulated by the aperture of the lens." They show that in the same lens a stop having half the diameter of another works at one-forth the speed because it passes a light cone which has one-fourth the volume of the larger stop. All this is correct, but they would have made their mental picture yet plainer if they had emphasized the fact that the true seat where the intensity is produced by this light, is the individual salt grains in the sensitive film. They probably did not do this because they considered the matter already understood by their readers, since it is commonly known that the F/values of lenses are computed relative to a point on the film. But in explaining the variations in speed when different focal length lenses are used, they say that "the volume of light alone does not determine the speed," explaining that — "As the light travels away from the lens, its intensity is diminished." They illustrate this weakening of the light in their Figure 21, which figure is in fact, the same in principle as the common point source illustration in the school physics books, which show lines spreading from a single point on a candle flame. That is to say: After correctly building up the true and basic idea that the working speed of a lens is "regulated by the aperture of the lens," they proceed to annihilate the lens apertures completely, illustrating them in the figure mentioned, as a point. In other words: In order to prove their problem they first completely ignore the factors of that problem. I would ask Messrs. Melson and Lawrence (when dealing with lenses of different focal length) if the independent salt grains have ceased to be the recipient of the energy which plays upon each from the whole stop area. I hold that no exposure was ever made in a camera except by the action of light from the whole stop area as it converged upon the independent grains of salts in the film. When they ignore the apertures and compare them to a point, as they do in their Figure 21, they destroy the problem completely by taking away volume, the working power which they had rightly developed in the first part of their article as a fundamental factor of lens speed. They state: "If the focal length — the distance of the plate from the lens — is different, the volume of light alone does not determine the speed . . ." They then, at some length, proceed to develop the fact that with altered focal length, the speed varies inversely to the square of the focal lengths. They had no need, however, to apply this law to the spreading of the light from a point, because it applies just the same to the change in the volume of the light as it converges from the whole stop to a single grain of salts on the film. Thus: If a stop one inch in diameter in an eight inch focal length lens (F/8) be used in photographing some small object exact size, the lens and bellows must be drawn out another eight inches from the film to do so. The cone of light which now converges from that same stop to a grain of salts on the film will be sixteen inches long instead of eight and this stop becomes, in fact, F/16. The idea of the light converging to a point from the stop area is preserved and the same mathematical law applies as it does for the light spreading from a point. It is seen that the volume alone modifies the lens speed in spite of their assertion to the contrary. There is no other way by which a grain of salts can be affected, other than by the influx of energy to it. This is true in open nature as well as in lenses. No molecule of matter can possibly be illuminated by light which issues from a point for the simple reason that it cannot receive more than a single ray of light from such a point. Neither visibility nor measurable intensity was ever created by a single ray of light. This point source theory lies entirely outside of the functioning of light in nature. It is fantastic and completely theoretical and inapplicable to nature. It includes no possible problem of intensity because it ignores quantity in light sources. Intensity is created AT points (molecules) not BY points. In photography, for example, observe the functioning of quantities: A faster film requires less exposure than a slower one. (Duration of time.) A brighter subject requires less exposure than a weaker one. (Variations in the degree of brightness, as of surfaces to be photographed.) A larger stop requires less exposure than a smaller one. (Variations of volume or solid angle in the light pencils which converge upon the salt grains.) These factors of brightness cannot be ignored and preserve the problem. On the contrary they must be assembled into a studiable plan for use in the schools. Flammarion said that the intensity on the several planets as created by the sun, depended solely on the "extension" or size of the sun in the sky of the different planets. This is the true law, -which Melson and Lawrence developed at the beginning of their article in dealing with the apertures of a lens. This also is the law made clear by the Rumford photometer, where the flames are dealt with as entities instead of points. This Rumford photometer is illustrated in Millikan and Gale's "A First Course in Physics" (Edition of 1906, page 393). Flames are considered as functioning entities. These authors also speak purely and correctly in quantities in discussing the matter of "Candle Power." Yet they use the common point source illustration on page 392, where the flame (the cause) is ignored as an entity. This is done without any explanation to students regarding the contrary nature of these two plans of study. The result is the public ignorance of light which we know exists today, with the almost universal practice of snap shooting in amateur photography. Now the truth is that, regardless of the distance of the areas from the flame, these illuminated surfaces are made up of molecules of matter each one of which must be independently illuminated by the light which comes to it from the WHOLE flame, precisely as shown by Millikan and Gale in their discussion of "Candle Power" and in their use of the Rumford photometer. I would respectfully inquire of Messrs. 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