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CINEMATOGRAPHIC ANNUAL
Steinhcil also discovered another principle of very great value in the design of photographic lenses; namely, the practice of introducing into one component a large overcorrection of some aberrations and compensating them by suitable amounts of undercorrection in the other component with the advantage that he thereby reduced the uncorrectable residual amount of aberrations which must always exist in any lens. This principle was enunciated in U. S. Patent No. 241438, granted to Steinheil on May 10, 1881.
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Fig. 5 — Steinhcil's Aplanat (Rapid Rectilinear)
•All of these lenses, however, exhibited astigmatism, curvature of field or both, generally both. The first objective to be simultaneously corrected for astigmatism and curvature of field is described in U. S. Patent No. 444714, granted to Dr. Rudolph on January 13, 1891. This achievement was made possible by the perfection of types of glass which were not available to the earlier designers. One of the constructions described in this patent reached the very respectable speed of f:6.0. Other examples, however, were low speed lenses.
A period of intense activity in photographic lens design followed the invention by Rudolph and many different forms of lenses were proposed, some symmetrical and some unsymmetrical. In 1895, Harold Dennis Taylor, in U. S. Patent No. 540122, described a lens corrected for all the ordinary operations and composed of but three elements, two collective lenses and one dispersive lens mounted between them, separated by air spaces. This form which has become widely known as the Cooke Triplet is shown in Fig. 6. In 1903, U. S. Patent No. 72140 was granted to Dr. Rudolph covering the construction which has become world famous under the name Tessar. (Illustrated in Fig. 7). This lens forms an image of excellent quality over a field of 50° to 60° at a speed of f:4.5 which is fast enough for the needs of hand cameras and most other ordinary applications of photography. This construction was later carried to a speed of f:3.5 in lenses of shorter focal length, and this for several years represented the highest available speed in a lens corrected for astigmatism and curvature of field and for many years this was regarded as adequate for motion picture photography.