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Transmission Color in Camera Lenses
By PHILIP T. SCHARF
The color contribution of a lens has been defined in terms of its transmittance density at 400 and 700 mM. It is proposed that the difference in the densities at these two wavelengths be held to 0.05 ± 03 for motion picture camera lenses; 0.05 d= .05 for other camera lenses. To prevent the curve from being too highly inflected between these two points an additional requirement is that the minimum density between 400 and 700 m/x differ from the density at 700 m^t by no more than 0.04 density units. It has been found convenient to describe quantitatively the glass absorption by a term called "color index." A simple method of determining the combined effect of glass absorption and surface coating is outlined.
-L HERE HAS BEEN considerable interest in recent years in the subject of camera lens transmission. Prior to the advent of antireflecting coatings it was not unheard of to have light losses amounting to as much as 50%. With present-day coated lenses, however, losses greater than 10% are seldom encountered. In terms of lens aperture this means that the loss of light with a coated lens does not exceed J of a stop. While there may still be reason for occasionally investigating the overall light loss of a coated lens, other variables in the photographic process will generally mask this ^ of a stop. On the other hand, the color of the light transmitted by camera lenses
A contribution submitted June 17, 1952, by Philip T. Scharf, Process Development Dept., Hawk-Eye Works, Eastman Kodak Co., Rochester, N.Y.
has caused much less interest, but in present-day practices it is probably more important than the overall white light transmittance. The increased interest in color photography is responsible for the importance of this color factor. Interchangeable lenses on cine cameras have made it possible for the photographer to perform a very critical test for the uniformity of color in his lenses. In this case the same scene, the same film, and the same processing variables are maintained while only the lens is varied. This means that if the same exposure is used, the only variable is the color of the lens. We wish to center our attention on this subject of color variations occurring in lenses, and to investigate methods of minimizing this variation.
There are two distinct factors contributing to the color of a lens. The first is the color of the glass itself. This
September 1952 Journal of the SMPTE Vol. 59
191