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Exploring Photography
Figure 3-18. The focal length or range of focal lengths is marked on each lens body at least once.
Focal length affected by digital sensor size
The focal length marked on the lens
may not be the focal length exhibited by
your digital camera. The size of the sensor
affects the focal length, typically extending
the focal length. The crop factor is the ratio
of the diagonal length of a camera’s sensor
to the diagonal length of a 35 mm frame. See
Figure 3-19. To fi nd the focal length of a lens
that is attached to a camera, multiply the focal
length printed on the camera by the crop fac-
tor. For example, a 50 mm lens mounted on a
camera with a crop factor of 1.6 will extend
the lens to 80 mm. See Figure 3-20.
Lens Angle of View
The focal length indicates the angle of
view of the lens and the image size that will
appear on the fi lm or sensor. Angle of view
is the largest angle of light rays that will
pass through the lens and form an image of
acceptable quality on the fi lm or sensor. See
Figure 3-21.
Selecting a Normal Lens
A normal lens produces an image about
the same, as it would appear to the human
eye. The angle of view is almost the same as
is seen by the eye.
To determine which lens is a normal lens
for a particular camera, fi nd the diagonal dis-
tance of the fi lm frame or the sensor. This is
approximately equal to the focal length of the
camera’s normal lens. See Figure 3-22. The
frame size of a 35 mm camera is 24 mm ×
36 mm. The diagonal of the frame is 43 mm.
A lens with a focal length in the range of
40 mm to 55 mm is considered normal for a
35 mm camera. The 50 mm is a popular nor-
mal lens for a 35 mm camera, but there are a
few manufacturers making a 43 mm lens. See
Figure 3-23.
A 2¼″ × 2¼″ (6 cm × 6 cm) camera has
a frame diagonal of 85 mm. Any lens in the
range of 75 mm to 85 mm is considered the
normal lens for this camera.
A digital camera with a 24 × 16 mm sen-
sor has a sensor diagonal of about 34 mm. A
35 mm lens is the normal lens for this camera.
Image Size
Image size is the size of the subject pro-
duced on the fi lm or digital sensor. The image
size can be changed by moving either the
camera or subject, or by using a lens of a dif-
ferent focal length. A longer focal length will
increase the image size and narrow the angle
of view. A shorter focal length will decrease
the image size and expand the angle of view.