112 Better Available Light Digital Photography
or one second. It can also be 1/25 of a second or 1/4000 of a
second. This number is the length of time the camera’s shutter
stays open to make the exposure. It’s the amount of time that light
passes the shutter curtain and exposes the image onto camera’s
sensor. The aperture is the size of the diaphragm opening in the
lens that controls the amount of light reaching the sensor. A good
analogy can be made with water pipes. The aperture is the diam-
eter of the pipe, whereas shutter speed determines how long a
valve is open, allowing water to fl ow through the pipe.
The amount of light getting through and the amount of time that
light is allowed through are equally important to proper expo-
sure. Both must work hand in hand for this proper exposure. The
good news is the aperture and shutter speed have a one-to-one
relationship. If you adjust the shutter by one full speed, then you
must adjust the lens aperture one full stop, and vice versa.
Shutter speeds are usually measured in fractions of seconds, with
the standard sequence having an approximate 1 : 2 ratio between
each one. The difference between two adjacent shutter speeds
on a dial or between aperture values on a lens is often called a
stop or a full stop. Most cameras allow adjustments between
these stops in one-half or one-third increments. Let’s say the
correct exposure for a specifi c shot is 1/1000^ second at f/2. In
this case, slowing the shutter speed to 1/500 second requires the
aperture to be closed down to f/2.8. A shutter change to 1/250
second requires the aperture change to f/4.0, and so on.
Let’s suppose your photographic assignment is in a low-light situ-
ation. Perhaps it’s indoors at a presentation, a school function, a
wedding reception, or an athletic event in a gymnasium. Your eye
automatically adjusts to the lower illumination and thus your
brain perceives the scene as adequately lit. As a photographer, you
know how deceiving this can be. Compared to working outdoorsWe generally think of fast lenses
for action-packed coverage of
sports, from loud auto racing to
Olympic events. These specialty
lenses are just as important,
however, for many quiet, serene,
everyday events. A fast telephoto
zoom lens produced a threefold
benefi t for this photograph. The
women in the prayer circle were
not interrupted or disturbed
because the 70–200 mm f/2.8
lens allowed the photographer to
stay well back out of their “space.”
The fast f/2.8 lens opening pro-
vided shallow depth of fi eld so
the hands in foreground are in
sharp focus and the arms and
hands in the background are out
of focus. The wide aperture also
allowed the image to be taken
with the available light in the
room, eliminating the need for a
fl ash that would have totally
ruined the moment photographi-
cally and spiritually. © 2006
Barry Staver.