37
Chapter 3 Lenses
Figure 3-17. This is a list of common lenses,
organized by focal lengths.
Models of lenses are drawn on computers
and evaluated before they are manufactured.
In the past, the calculations were made by
hand. It often took years to design a lens.
Finished lens elements are precisely fi t-
ted into the lens barrel. The lens barrel is
carefully fi tted to the camera body.
Focal Length of a Lens
All lenses have a focal length. The focal
length of a lens is the distance between the
lens and the focal plane when the lens is set
to infi nity. See Figure 3-16. The fi lm or sen-
sor surface is called the focal plane. Usually,
focal length is described in millimeters (mm),
but some lenses use centimeters (cm) and
inches. See Figure 3-17 for a list of some com-
mon focal lengths. The focal length or range
of focal lengths, of a zoom lens, is marked on
the lens. You can fi nd the markings on the
front of the lens, side of the lens, or multiple
places. See Figure 3-18.
Wide angle
14 mm
20 mm
24 mm
28 mm
35 mm
50 mm
85 mm
100 mm
135 mm
200 mm
300 mm
400 mm
500 mm
600 mm
800 mm
Telephoto
Infinity
∞
50 mm
Film surface
or digital sensor
The area the camera lens
sees when focused on 35 mm film.
Film image size is 36 mm × 24 mm.
Figure 3-16. Focal length is the distance between the lens and focal point, fi lm, or sensor surface when the
lens is set to infi nity.