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long-focal-length lens and one sometimes sees references made to the telephoto ratio of a wide-angle lens.

Two significant advances characterize the newer types of telephoto lenses, particularly those used for 35-mm SLR cameras. The first is the use of small internal groups for focusing, sometimes in conjunction with the front group. This feature has also led to significant improvement in the performance of these lenses with change of conjugate. This has been a problem with telephoto lenses, particularly with respect to attaining close focus with good optical quality. Internal focusing of a long-focal-length lens also has considerable advantages in terms of mechanical simplicity because a smaller mass is being moved over a significantly shorter distance.

The second advance is the employment of optical glasses having anomalous dispersion for the correction of chromatic aberrations. These newer glasses have anomalous dispersion characteristics similar to those of calcium fluorite, but with physical and chemical properties that make their use practical. These glasses are still expensive and more difficult to use than ordinary ones, but they do offer significant advantages in terms of reducing the chromatic aberrations that otherwise severely limit the imaging potential of all long-focal-length refracting optics. Typical available versions of these glass types are the FK Schott, FCD Hoya, FPL Ohara, and PFC Corning series of glasses.

These design types offer outstanding optical correction together with remarkable specifications, resulting in considerable size and cost. Commercial embodiments include 300-mm F/2 and 400-mm F/2.8 for 35-mm. They are widely used for sports and wildlife photography. Since secondary color increases as the front vertex distance is reduced, it is advisable to regard excessively short all-refractive telephotos with sonic caution. Refer to Figs. 12 and 13.

Zoom Lenses

Zoom lenses have evolved significantly in the past twenty years. In the early 1970s, there was basically only the classic four-group type of zoom lens. This four-group zoom has two moving groups between a front group used only for focusing and a stationary rear ("master") group. This type is still found on consumer video cameras. Figures 14 and 15 show a variation of this form with the rear group also moving for zooming. The master group could often be changed to yield a different zoom with the same ratio over a different range.

The second basic form, originating in the mid 1970s, was the two-group wide-angle zoom, typically 24 to 48 mm and 35 to 70 mm for the 35-mm format. Both the front negative group and the rear positive group move for zooming, and the front group is also used for focusing. This lens type has an inherently long back focal length, making it eminently suitable for the SLR camera. See, for example, USP 4,844,599. The maximum zoom range is about 3 : 1.

In order to achieve lens types such as a 28- to 200-mm zoom for 35-mm, new ideas had to be employed. The resulting lenses have up to five independent motions, including that of the diaphragm. These degrees of freedom allow for the location of the entrance pupil to he near the front of the lens at the short-focal-length position and also for the exit pupil to be located near the rear, particularly at the long-focal-length setting. These conditions result in acceptably small size. The extra zooming motions permit a large focal-length range to be achieved without any one motion being excessively long. There is a constant struggle in the design of these zooms to minimize the diameter of the front of' tile lens. This is not only to reduce size and weight, but also to permit the use of acceptably small filters. Some designs do have problems with relative illumination at the wide-angle end.

In the past, these lenses have been focused either by moving the front group or by moving the entire lens, the latter option leading to the so-called varifocal zoom. However, more recent developments in miniature electromechanical and autofocusing systems have led to the evolution of extended range zooms in which the distinction between a focusing