Catadioptric Lenses

Catadioptric Lenses

Normal camera lenses are refracting instruments, with all their glass elements being lenses. For especially long telephoto use there is another type of construction that in essence is a reflecting telescope with a "folded" light path that incorporates one or more refracting elements (called correcting lenses) to yield a flat image in the film plane. These lenses are called mirror optics or, more correctly, catadioptric optics. A pure reflecting system is unusually free of most optical aberrations, but it cannot produce a flat image, its image naturally being a portion of a sphere. Thus, correcting lenses of some type must be introduced.

These designs have a number of advantages, in addition to their potentially fine optical quality. They are unusually compact and light in weight, they focus rapidly over considerable distance ranges, they are usually of very long focal length, and they frequently have a fairly wide aperture for such a focal length (although they are slow compared with normally constructed lenses of short to medium focal length). Their overall lengths are as short as 7-8 inches (17.8-20.5 mm). Their focal lengths usually range from about 500 mm to 2000 mm and their relative apertures range from about f/4 to about f/11 (a very few may have operating apertures as low as f/16 or f/22).

The extreme image magnification and typically great working distances of these very long focal length lenses introduce two major limiting factors in obtaining good images:

  1. Any movement or vibration of the camera will blur the picture.

  2. Convection currents and other air movements between the lens and the subject will badly degrade the image.

Movement and vibration must be very carefully controlled to eliminate most or all such effects. Atmospheric conditions may require that most such work be done in the early hours of the day, when air movement is at a minimum.

Catadioptric optical systems have certain disadvantages. These include:

  1. Photographs made with them display a very shallow depth of field, because of the combination of high image magnification and a fixed, rather wide aperture.

  2. Exposure control is a little abnormal, in that the aperture is fixed, and control is exercised by varying the shutter or ISO and film speed; or, in a few cases, by using neutral-density filters. Only rarely can exposure control be exercised in increments of less than a full stop.

Nevertheless, remarkably good photographs of distant people, and of small, hard-to-approach wildlife, can be made with catadioptric lenses.

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