This occurs because exposure systems think that no mat-
ter how bright or how dim something is, it is 18% gray
(which is considered a medium gray). In these pho-
tographs, you can see white, gray, and black sheets of
paper in their true colors when they are all in the same
photo. But when each is photographed so that it is the
only object measured by the exposure meter, the cam-
era’s exposure automatically is set to render the three
sheets of paper as the same medium gray.3
The other common alternative is spot metering, which gives
the photographer greater ability to expose the part of the
scene that is most crucial. Illumination is read from only a
small circle in the center of the screen, allowing the photogra-
pher to expose for the lighting on a cheek that might be sur-
rounded by dark hair and a beard that would otherwise
overwhelm the light readings. When the crucial subject matter
is off-center, some cameras have the capability to make small
areas on different parts of the image the spot for purposes of
metering. For those without such cameras, you can use the
shutter lockdescribed in the box below.5
The Half-Push
When the most important element of a photo is not in the center of the
frame, most cameras have a shutter lock feature that lets them focus and get
an exposure reading for their picture by aiming the center at that important
element. Then, by pushing the shutter button only halfway, the autofocus and
autoexposure settings are locked. The photographer then reframes the pic-
ture with that key element away from the center and presses the shutter but-
ton the rest of the way.To overcome the perils of using an average of an entire
scene’s illumination, better digital cameras have alterna-
tive ways of measuring light that can be chosen from a
menu displayed in the camera’s LCD screen or by one
of the camera’s control knobs or buttons. The first alter-
native iscenter-weightedmeasurement. It meters the
light in an area that amounts to about a tenth of the
total photo area. As the name implies, that tenth is
located in the center of the screen on the theory that
that’s where the most important part of the scene is.4
CHAPTER 5 HOW DIGITAL EXPOSURE SIFTS, MEASURES, AND SLICES LIGHT 71