42
Exploring Photography
pass through to the fi lm or sensor. This per-
mits satisfactory photographs to be taken in
dim light.
Closing the lens aperture by one f-stop
(f/2 to f/2.8, for example) reduces the light
reaching the fi lm or sensor by one-half, as
shown in Figure 3-28. Reducing the size of
the aperture is called stopping down. The
same f-number on any lens of the same focal
length will allow the same amount of light to
reach the fi lm.
Mounting Lenses on a
Camera
The lens mount attaches the lens to the
camera body. This feature permits the pho-
tographer to change lenses rapidly. There are
two main types of mounts: screw mount and
bayonet mount.
A screw mount lens uses threads to
attach it to a camera body. See Figure 3-29.
Several revolutions are needed to seat the lens
Aperture
Figure 3-25. The aperture of a lens is changed with
a diaphragm-like device between the lens elements.
The amount of light passing through the lens to the
fi lm or sensor is controlled by making the aperture
larger or smaller.
Figure 3-26. Aperture size is specifi ed by f-stops.
They appear as f-numbers on the lens barrel.
Figure 3-27. Many lenses made for digital cameras
lack a mechanical aperture ring. The aperture is
controlled by internal motors. (Sigma)
The “f” stands for fraction. A larger f-num-
ber has a smaller aperture opening. For exam-
ple, an f/8 (1/8) aperture is smaller than an
f/4 (1/4) aperture.
A lens with a maximum f-number aper-
ture opening of f/2.8 is considered a fast lens.
A fast lens allows a large amount of light to