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How Your Exposure System Works...9
All digital camera exposure systems operate on the same general principles.
A meter continuously measures the light reflecting from the subject and uses
this measurement when you press the shutter button halfway down to calcu-
late and set the shutter speed and aperture.
Your camera’s meter measures some or all of the light reflecting from the part
of the scene shown in the viewfinder or on the monitor. The coverage of the
meter (the amount of the scene that it includes in its reading) changes just as
your viewfinder image changes when you change your distance to the scene
or change the focal length of the lens. Suppose you move close or zoom in and
see in your viewfinder only a detail in the scene, one that is darker or lighter
than other objects nearby. The suggested aperture and shutter speed settings
will be different for the detail than for the overall scene.meter AVerAging And middle grAy
Your exposure meter doesn’t “see” a scene the same way you see it. Its view is
much like yours would be if you were looking through a piece of frosted glass.How Your Meter Sees
scenes as if it were
looking at them through
a piece of frosted glass.
It doesn’t see details,
just averages.Every scene you photograph is something like a checker board pattern on the
buildings (left), but often more complex. Portions of most scenes are pure
black, pure white, and every possible tone in between.
The camera’s exposure meter and exposure control system can’t think. Re-
gardless of the scene, its subject matter, color, brightness, or composition,
the meter does only one thing—it measures the average brightness, or how
light or dark the scene is. The automatic exposure system then calculates
and sets the aperture and the shutter speed to render this level of brightness
as “middle gray” in the photograph. Most of the time this works very well
because most scenes have an overall brightness that averages out to middle
gray. But some scenes and situations don’t average out to middle gray and
that’s when autoexposure will lead you astray. Let’s see why.Where you see a
checkerboard-like
pattern (top), your
camera sees only an
average gray (bottom).Click to explore how
your exposure system
“sees” a scene.