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:
Any movement or vibration of the camera
will blur the picture.
Convection currents and other air
movements between the lens and the subject will badly degrade the
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:
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.
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.