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
l-35
