The international photographer (Jan-Dec 1935)

Sixteen The INTERNATIONAL PHOTOGRAPHER November, 1935 Historical Background of the Speed Panchro Lens C \tSD\CKTc.D * Manager Technical Service, Bell & Howell Co., Chicago. •■01 r-01 COOKE SPEED PANCHRO1" LENS ABERRATION CURVES FOR C.D.F.&O jHE CINEMATOGRAPHER is undoubtedly more critical in his judgment of the qualities of any photographic lens than is any other worker. The very nature of his work compels him to be extremely careful in the selection of the lens for his work. For instance, when we realize that the area of each individual frame of motion picture film is less than one-half of a square inch, we can readily perceive the enormous magnification involved in showing this on a theatre screen. For instance, with a thirty-foot screen the magnification in area is 16,250 times. Analyzing the matter further, we have looked at it this ■way. Using the standard two-inch lens on the average scene, the camera will be somewhere about twenty feet away from the actors. Figuring that an actor's face is approximately six inches in diameter, this means that the face is registered on the film within an area of 1/20-inch diameter. Going further and considering that the features, say for instance the eye, being about one inch in size, would occupy an area of only 1/120-inch in diameter. We must realize that the individual grains of the motion picture film, while very small, do have a discreet size. We can usually figure that they are not very much smaller than .001 -inch diameter. Therefore, when we figure that an actor's eye occupies a portion of the film only 1/120 inch in diameter, we can see that this image is made up of approximately ten or so grains of developed silver. These facts and figures are offered to dramatize the striking and critical refinements of the motion picture lens which must resolve the finest detail as sharply as can be resolved on the film used by the cameraman, so that when the print is finally projected on the screen, the individual features, nuances of expression, will be correctly delineated. Not only does the motion picture lens do this, but it is corrected to limits even beyond the resolution of the best films available at this date, remarkable as they are. These lenses are also corrected for color photography so that the cinematographer who has such modern lenses at his disposal is equipped to handle any new developments that may be brought out and to get the utmost out of them from an optical (or chromatic) standpoint. The following brief historical resume of the development of these lenses is offered to show some of the complex research investigations which have been made by various optical geniuses and upon whose work and by whose work it has been possible for the cameramen to have at their disposal such remarkable lenses as they are now using. Basically, the modern camera lens is an elaboration of the ordinary magnifying glass, although it is as far ahead of the magnifying glass as the modern airplane is beyond the kite. Most of the readers of the "International Photographer" are familiar with the conventional optics as outlined in the regular physics books, but it will not harm to list the various aberrations of a plain lens, which have to be corrected by combining two or more lenses of different characteristics. It is this knack of being By R. Fawn Mitchell* able to combine different optical glasses in different combinations of curves, spacing, etc., that marks the difference of a photographic lens from a magnifying glass. First of all, the lens designer is handicapped in that he has to use spherical surfaces on his lens with the result that spherical aberrations have to be corrected by various expedients. Spherical aberration means that rays from the margin of a lens do not focus at the same place as those passing through the central portion or intermediate portions of the lens. If it were possible to use non-spherical curves on a commercial basis, many of these problems would be simplified. Unfortunately, however, the generation of parabolic curves does not lend itself to production methods, as it is more or less essentially a skillful hand job. Spherical curves, on the other hand, can be made with extreme accuracy by machine under regular production conditions. The second major trouble facing the lens designer is that of chromatic aberrations, which means that different colors focus a* different positions along the axis. Most cinematographers are able to appreciate the importance of this correction because within recent years they have had to scrap a lot of their old lenses tc use the new type lenses corrected for the combination of panchromatic film and incandescent lighting wherefor the combination of panchromatic film and incandescent lighting ■wherein the lens correction has been arranged so as to focus the red rays at the same position as the blue rays which will be enlarged upon at a later point in the article. The third principal aberration is distortion, which, as the name implies, means that an ordinary lens tends to focus straight lines as curves, especially at the edge of the picture. Closely allied to distortion is curvature of the field, which means that the lens may focus the center of the image sharply on the center of the frame, but the edges would be out of focus. If the film or lens was moved forward or backward to bring the edges in focus, the center would be out of focus. As long as film has to be located flatly at the aperture, the importance of this correction is obvious. Beyond these major corrections, there are additional errors which have to be compensated for in order to get highly corrected lenses of large aperture. Historically, as will be outlined further, lenses of satisfactory correction of the first four aberrations were developed that were limited to speeds of approximately F 8 or F 7. It was only upon the introduction of new optical glasses by Abbe & Schott and Jena, when it became possible to handle the correction of astigmatism and other complex corrections. Astigmatism, as the name implies, describes the effect of lenses upon rays coming from a point away from the axis. In other words, the image of a cross located in the edge of the field would be focused by an uncorrected lens as either a horizontal line sharply with a vertical line out of focus, or vice versa. Somewhat similar to astigmatism is the trouble called coma. In other words, the image at a point located near the edge of the field would be focused as a spot with a tail of lesser density almost identical in appearance to a comet. Hence the name. A still further difficulty that has to be corrected in a modern F 2 Cooke Speed Panchro Lens Please mention The International Photographer when corresponding with advertisers.