American cinematographer (July 1922)

4 THE AMERICAN CINEMATOGRAPHER July 1, 1922 tlfcobern Senses m SECTION THREE By Karl Brown, A. S. C. NOTE — The American Cinematographer gratefully acknowledges the valuable assistance of Mr. M. C. Williamson, of the Woltensak Optical Company, for data and diagrams; and the data supplied by Mr. Henry T. Branstotter of the Hanovia Chemical and Mfg. Co. The Anastigmat The modern anastigmatic lens is a most striking example of the triumph of sheer intellect over seemingly physical impossibilities. Nothing in the history of science, not even the airplane, or the radio, is a greater monument to human invention than the anastigmat. It is really and truly a scientific miracle. The achievement of the modern anastigmat is the result of over two-thirds of a century of hard work and concentration. During this time, the ideal of what a lens should be, changed with the necessities of contemporary conditions. The first ideal was that of great speed, with which to overcome the difficulties of photography with the very slow emulsions of that day. Petzval, one of the most brilliant of all opticians, in his attempts to reach this ideal, produced and marketed the famous Petzval Portrait Lens, of the then tremendous aperture of F.3.2. in the year 1840. The Petzval lens was a collossal achievement, and one which gave photography a great forward step. The development of the Petzval formula by Dallmeyer and Zinke-Sommer gave photographers an improved lens with a greater aperture, F.2.3., and the day of extremely prolonged exposures was over. The Petzval Portrait Lens, in a modified form, is still a favorite. The Wollensak Vitax, F.3.8. is such a lens. The faster emulsions were made. The wet plate, then the dry plate, contributed more and more to the practicability of photography, and as the emulsions improved, less attention was paid to speediness in lenses, and the long and discouraging struggle for perfect correction began, an effort which owes its success to the development of that modern marvel, optical glass. The advent of optical glass gave an immense impetus to lens making. The lack of that very material had been the stumbling block. It is one thing to calculate a lens, and another thing to make one. With the limited glasses at their command many things were practically impossible, although theoretically feasible enough if they could but get the proper glass. The new glass was produced and developed to the point where it was possible to obtain almost any desired qualities, thus opening the way for a new era of lenses. With the versatility of the new glasses at their command, the next step was the production of high speed, fully corrected anastigmatic lenses, an ideal which was quickly reached in a number of brilliant examples, first of which were the lenses of Carl Zeiss. This ideal of perfect definition reflects itself in all photography of that day, with the exception of a small group of misunderstood artists who were producing laughed-at soft images which are the despair of modern workers. An ideal portrait was one looking like a statue; one flattered the photographer by commenting on the perfection with which every hair was registered. This ideal held force until it dawned upon the photographic world that art was not necessarily a matter of optically perfect definition; bpf that it might be possi ble to make a good picture without this cherished precision. The few workers of the advance guard found themselves held in a little less scorn than before, and once the idea of soft-image got really under way, the whole pictorial world began turning out a few soft image pictures, and a very great many soft focus ones. There’s a vast difference! Now it is difficult to find a sharp portrait anywhere. Even mechanical devices are advertised with soft focus photographs. This last change of ideal brought about the development of more lenses than any of the others. It is difficult to produce a high speed lens, and still more difficult to produce a high speed, fully corrected lens, but any old lens which will not give a perfect image may be properly and safely classed as a soft focus lens. Probably the most satisfactory of our present soft focus lenses is the simple spectacle lens, wholly uncorrected. This last ideal of what a perfect soft image lens should be has not yet crystallized to the point where makers can concentrate on a given objective. Too much variance of opinion makes this impossible. Soft image photography is still in the formative stage. This change of ideal was most fortunate for the cinematographer. He needed, particularly at the beginning of the art, speed, and definition, and now, in the later and more artistic stage, the beauty of the soft image. The first two were waiting for him; the last he must help develop. The modern motion picture anastigmat is, with the possible exception of the Process Apochromat, the most perfect photographic objective in the world. The shortness of its focal length in relation to the comparatively great distance of the object being photographed brings about a highly desirable optical condition. To photograph an object at a distance of ten feet with a twoinch lens is equivalent to photographing an object fifty feet away with a ten-inch lens, as far as the optics are concerned. Hence the great “depth,” properly “depth of field,”’ of the usual motion picture lens. This condition of work is responsible for the very fine performance of motion picture lenses, as compared to larger lenses of the same make. The subject of depth of field is one very commonly misunderstood by a large number of photographers. The writer has heard innumerable assertions concerning the superiority of a certain make of lens in its great “depth” as compared with other lenses. Such arguments have no foundation in fact. Depth of field has nothing to do with the formula or type of lens; the determining factors are size of aperture and focal length. Two dissimilar anastigmatic lenses of the same aperture and focal length are equal in depth of field. The depth of field increases as the size of the stop is diminished, and becomes greater the more removed the object focused is from the lens. The exception to this rule is in soft lenses, which show a great increase in depth of field over an anastigmat of the same aperture and focal length, and this varies with the formula. Some soft lenses show an increase of several hundred per cent, greater acceptable depth of field over a similar anastigmat. A lens "carries” farther back of the point focused on than in front. For instance, any two-inch lens