A lens with a focal length greater than about 58mm for a 35mm camera is a long-focal-length lens. Most modern, long-focal-length lenses are called telephoto lenses because of their compact design. At one time, long-focal-length lenses were essentially a lens at the end of a long tube. A 500mm lens was spaced 500mm from the film, and so on. However, by incorporating other glass elements, the light passing through the lens can be modified (fig. 1-35). This permits the lens barrel to be physically shorter than the lens actual focal length-an arrangement known as telephoto.

Figure 1-36. Reflecting telephoto lens.

The overall physical length of a telephoto lens is usually only about one half of its focal length. A basic, long-focal-length lens must be placed one focal length away from the film if it is to form an image of a subject at infinity. In the case of a telephoto (or mirror) lens, the lens-to-film distance is reduced considerably while still retaining the effects of a long-focal-length lens. Thus a 1000mm telephoto lens rear element may only be 500mm away from the film when the lens is set at infinity.

Those 35mm camera lenses that range from about 85mm to 135mm are good for shooting pictures of people. They allow you to shoot from about 6 feet away and still fill the frame with the subject's face. Six feet from the subject is a good working distance. It is not too close for comfort, and it is not so far away that intimacy is lost.

Telephoto compression is the apparent compression of perspective. A telephoto lens does not compress perspective; it only appears that way! Remember, perspective does not depend on the lens being used, but on the position of the camera.

So then, how does a telephoto lens produce the effect of compressed perspective? Several factors are involved:

A telephoto lens is used from farther away to obtain the same size image that would be produced by a shorter lens at a closer distance. The more distant camera position produces a flatter perspective. But, because the long lens magnifies the subject, it still produces a normal size image. Thus the looks are flatter than expected.

The distance from which the print is viewed also has an effect. An X-times enlargement should be viewed from X-times the focal length of the lens used to make the picture in order for the perspective to appear natural. Therefore, a 6X enlargement of a negative shot with a 50mm lens should be viewed from 6X 50mm = 300mm or 12 inches, while a picture made with a 500mm telephoto lens and enlarged 12 times should be viewed from20feet(12 x 500mm=600 x 0.04 = 240 12 = 20 feet). (Note: To convert millimeters to inches, multiply the known millimeters by 0.04.)

A reflecting telephoto lens, the so-called mirror lens, has folded up optics. It uses internal mirrors to reflect the light twice. This enables the lens barrel to be much shorter, but because of the mirrors, it must also be much broader. As shown in figure 1-36, light that enters the lens through a glass plate is converged and reflected back by a concave mirror at the back of the lens. This reflected light is directed to a small backward-facing

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Figure 1-37. Out-of-focus highlights caused by a mirror lens.

mirror lens element at the center of the front glass plate. In turn, the mirror lens reflects the light back through a hole in the concave mirror to a focus on the film.

Mirror lenses have the advantage of long focal length, relatively short physical size, and large aperture. But they also have disadvantages, the main one being that a diaphragm cannot be used and the lens must always be used at maximum aperture. Therefore, exposure must be controlled by the shutter alone or by the use of neutral density filters, or both. Because of this aperture disadvantage, mirror lenses have limited depth of field. Another disadvantage is that out-of-focus highlights record as rings of light (fig. 1-37).