The digital processor’s job of finessing aperture and shutter settings is simplified
by the use of exposure values (EVs). These are numbers assigned to different com-
binations of aperture and shutter that result in the same exposure. When, for
example, the aperture closes down one more stop, the exposure remains the
same provided the shutter stays open longer. The new combination causes the
same amount of light to fall on the image sensor as did the previous one. This
chart shows the range of exposure equivalents.Most digital cameras provide specialized settings, often referred to as scene
modes, for situations that don’t fall into the standard exposure modes. Some
cameras provide more than two dozen such scene modes, including one mode for
making people look as if they’ve lost several pounds. Here are a few of the more
common:- Portrait—Opens the aperture as wide as possible for a minimum depth-of-field
that eliminates distracting backgrounds that could draw attention away from the
subject of the portrait. - Landscape—Sets the aperture as small as possible for maximum depth-of-field.
- Close-up—Also sets for maximum depth-of-field because depth-of-field naturally shrinks
as the distance to a subject gets shorter. - Action, or Sports—The shutter speed is set as fast as possible to freeze action.
- Night Portrait, or Twilight—The shutter speed is set to slow, and the flash is brought into action.
The flash illuminates the foreground, and the shutter remains open to pick up some of the background.
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After a camera’s photodiodes have measured the light coming from the subject and the camera has sent that information to a microprocessor, all
the information needed to obtain the perfect exposure is still not in. The processor must meld the light reading with other exposure controls the
photographer might have set in anticipation of the special requirements of this particular photo shoot. Finally, for better cameras, the photogra-
pher has veto power to step in and impose his own idea of how a scene should be exposed. Most cameras, however, let you take advantage of
specialized settings that take the worry—and thinking—out of photography.How Exposure Systems Balance
Aperture and Shutter
Any combination of
shutter speed and
aperture that meets
at the same diago-
nal line is an EV.
Here, an exposure
of f/8.0 at 1/60
of a second is
equivalent to
f/5.6 at 1/125
of a second. Both
exposures have an
EV of 12.
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74 PART 2 HOW DIGITAL CAMERAS CAPTURE IMAGES