Less-expensive digital cameras—called point-and-shoot (POS)
models—often have only one of the many ways of measuring light
that more-expensive cameras boast. It’s called full-frameand it
typically uses one photodiode device mounted next to the lens. On
better cameras, full-frame metering uses several photodiodes—as
many as 45—mounted in the path of the light on its way from the
lens to the shutter hiding the image sensor. Either type of full-frame
averages the intensities of light reflected off a subject to determine
a shutter speed and aperture that will produce an exposure that is
expected to render everything in the photo. But unless a scene is
evenly lit and contains only subjects with the same color value, full-
frame exposures are usually less accurate.1
In the photo here, for example, the
bright sunlight falling on the bricks
behind the boy riding in his car has
made the camera’s autoexposure fea-
ture overcompensate and shut down the
diaphragm too much. The result is
muddy shadows revealing little detail in
the most important part of the photo.2
How Light Meters See the World
The most complex part of a digital camera is its exposure system. It’s more than a photodiode measuring the light coming
through the lens. Among the most versatile cameras, the exposure system has more than one way to measure light and mixes
those measurements into a brew made of settings for the type of lighting; the sensitivity of the image sensor; and special set-
tings for action shots, fireworks, black-and-white, or even special effects such as sepia toning. That brew is siphoned to set into
action the diaphragm and shutter, all in the blink of an eye.
70 PART 2 HOW DIGITAL CAMERAS CAPTURE IMAGES