Your camera probably takes care of
exposure metering for you, setting either the shutter speed or the
aperture (or both) according to how much light the subject reflects,
and the ISO setting or speed of the film you're using. But often there
is misleading subjects around that can lead to exposure problems.
The key to anticipating such problems and minimizing the consequences
is to understand that the meter in your camera is essentially
unthinking - an obedient but stupid slave that obeys a series of simple
rules. As long as you know the rules, you'll get good exposures every
These are the two basic rules that most meters obey.
The tone of the subject averaged over a
selected area is neither light nor dark but an intermediate shade.
The middle of the picture is more
important than the corners. If you're faced with a subject that breaks
these rules, you must compensate and set exposure manually, or
intervene in some other way.
There are several ways of doing this. The
simplest is to find an area of the landscape that does obey the rule,
and meter from there. On a white sand beach, you might close in on a
grey rock, take a reading, set exposure manually or store the meter
reading in the camera's memory, then move out to recompose the picture
as you want it.
The other commonly-used method is to
meter the scene normally, then apply some compensation to take account
of the subject's unusually light or dark tones. In snow, for example,
you might choose to set the camera's exposure compensation dial to +1
or x2. This would have the effect of overexposing the film by one stop,
so that the snow comes out realistically white, not grey.
You can also use hand-held meter that
allows you to measure the incident light - the light falling on the
subject - rather than reflected light. By comparison, built-in meters
gauge the brightness of reflected light only. To use an incident light
meter, stand at the subject position and direct the diffusing dome at
the camera, not the light source.