JO1 Petcr D. Sundberg
302.19
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).
Variable Focal-Length Lenses
A variable focal length, or zoom, lens is designed so
the focal length can be changed by mechanically moving
the elements within the lens. The movement of lens
elements, in unison and in precise order, gives a smooth
change of image size while maintaining acceptably
sharp focus throughout the entire adjustment. The
simplified drawing of a zoom lens (fig. 1-38) illustrates
how the movement of elements within the lens can
change the focal length while maintaining correct
lens-to-film distance.
While only the shortest and longest focal lengths for
this particular lens are shown in the drawing, various
other focal lengths are possible.
The biggest advantage of a zoom lens is that you
have many focal lengths in one single lens. You do not
have to change lenses to use a different focal length.
Sometimes it is impossible to change your viewpoint to
improve a picture. But with a zoom lens you can zoom
in and out (change focal length) until you get the exact
image you want. One disadvantage is the extra bulk and
weight of the zoom lenses. There is also some loss in
picture quality when compared to the performance of a
fixed focal length lens. There are four basic types of
wide to telephoto zoom lenses for 35mm cameras:
Wide-range zoom lenses have focal lengths from
about 28mm to 80mm. They often take the place of fixed
focal-length lenses of 28mm, 35mm, 50mm, and 80mm.
Mid-range zoom lenses have focal length that do
not extend very far on either side of a normal lens focal
length. Mid-range zooms for 35mm cameras have a
1-36
