Light fall-off
Light from a flash unit mounted on the camera falls off,
or loses its effectiveness, very rapidly as distance increases.
This is evident in this close-up image of a bride holding a
posy of flowers. The subject’s hands and the roses nearest
the flash are brightly illuminated but even just a little bit
further back, the image is visibly darker. This is evident if
you look at the back edge of the wedding dress, for
example. The effects of this light fall-off can be greatly
minimized by using a light source that covers a larger
area—hence the very different lighting effect you get
when using bounced flash (opposite).
Modern electronic flash units are versatile and
convenient light sources, ideal for use when light
levels are low (and the subject is relatively close),
or when image contrast is high and you want to
add a little fill-in illumination to the shadow areas.
However, due to the intensity of their output as
well as their limited range and covering power,
obtaining naturalistic lighting effects and correct
exposure can be problematic.
Problem: Common types of problems that are
encountered when using electronic flash include
overexposed results—particularly of the foreground
parts of the image—and underexposed results—
particularly of the image background. In addition,
general underexposure of long-distance subject
matter is very common, as is uneven lighting, in
which the corners or foreground are less bright
than the center of the image.
Analysis: Modern electronic flash units have their
own light-sensitive sensor to measure automatically
the light output from the flash or the amount of light
reflecting back from the subject and reaching the
film or camera sensor. As a result, they are as prone
to error as any camera exposure meter. Furthermore,
the light produced by a flash unit falls off very rapidly
with distance (see above right and p. 80).
Overexposed flash-illuminated pictures are
usually caused by positioning the flash too close to
the subject or when the subject is the only element
in an otherwise largely empty space.
Flash underexposure is caused by the unit having
insufficient power to cover adequately the flash-to-
subject distance. For example, no small flash unit can
light an object that is more than about 33ft (10m)
away, and even quite powerful flash units cannot
adequately light an object that is more than about
100ft (30m) distant.
Uneven lighting is caused when the flash is unable
to cover the angle of view of the lens—a problem
most often experienced with wide-angles. And
another problem occurs when an attached lens or
lens accessory blocks the light from a camera-
mounted flash.
Solution: For close-up work, reduce the power of
the flash if possible. When photographing distant
subjects in the dark—landscapes, for example, or the
stage at a concert, using flash is usually a waste of
time and is best turned off. A better option is to use
a long exposure and support the camera on a tripod
or rest it on something stable, such as a wall or fence.
With accessory flash units—not built-in types—you
can place a diffuser over the flash window to help
spread the light and so prevent darkened corners
when using a wide-angle lens.

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