How the Flash Knows
When to Quit
The explanation in the previous illustration telling what takes place when the flash is
used may have been more complex than I promised it would. But it could have
been more complicated still. After all, it’s not enough simply to get a big flash of
light at the same time your shutter opens. It has to be the right amount of light,
and last for the right amount of time, so that the photograph is properly
exposed. To get the full picture—pun purely accidental—let’s step back a bit,
to the point at which the photographer is pressing the shutter button.88 PART 2 HOW DIGITAL CAMERAS CAPTURE IMAGES
Pressing the shutter button does more than cause the shutter to
open. First it signals that the photographer is happy with the focus
and that the photo is framed properly in the viewfinder. The circuits
and motors that control focus instantly freeze. Some cameras take a
reading of theambient lightlevels. These are the light from any
source, sun, candles, fluorescent lights—anything except the flash itself.
Many camera/flash combinations are able to concentrate on the light
levels in the background, where the flash will not reach. Based on the
type of exposure the photographer has chosen by selecting aperture or
shutter priorities, or scene settings for sunsets, action shots, or as many
as two dozen other possible scenes, the camera decides on the
best setting for the main
subject and the
surroundings,
information
it shares
with the
flash.Some flash units determine the most important part of a scene
from reading the distance the camera is focusing on when the
camera tells the flash the distance it’s focused on. Or a flash may
send out infrared beams, measure their return time, and apply
some photography common sense. There may be mountains in the
distance and an elk 20 feet away. But the flash’s microprocessor
brain sees only the elk. Because its light could never illuminate
faraway mountains, the flash settles on something within its
range—the elk—and sets its light intensity accordingly.2
1