The process of determining the amount of light that
is needed by the sensor for the required results is
called exposure control. Many digital cameras can
change sensi tivities to maintain practical camera
settings. For example, in low light, sensor sensitivity
increases (just as faster film might be used in film-
based photography) to allow you to set shorter
shutter times or smaller apertures.
It is important to control exposure accurately
and carefully, as it not only ensures you obtain the
best from whatever system you are using, but it also
saves you the time and effort of manipu lat ing the
image unnecessarily at a later stage.
Measuring systems
To determine exposure, the camera measures the
light reflecting from a scene. The simplest system
measures light from the entire field of view, treat ing
all of it equally. This system is found in some
handheld meters and some early SLR cameras.
Many cameras use a center-weighted system, in
which light from the entire field of view is registered,
but more account is taken of that coming from the
central portion of the image (often indicated on the
focusing screen). This can be taken further, so that
the light from most of the image is ignored, except
that from a central part. This can vary from a central
25 percent of the whole area to less than 5 percent.
For critical work, this sel ective area, or spot-
metering, system is the most accurate.
A far more elaborate exposure system divides
the entire image area into a patchwork of zones,
each of which is separately evaluated. This system,
commonly referred to as evaluative or matrix
metering, is extremely successful at delivering
consistently accurate expos ures over a wide range
of unusual or demanding lighting conditions.
As good as they are, the exposure systems found
in modern cameras are not perfect. There will be
times when you have to give the automa tion a
help ing hand—usually when the lighting is most
interesting or challeng ing. This is why it is so
crucially important to understand what exactly
constitutes “optimum exposure.
Optimized dynamics
Every photographic medium has a range over
which it can make accurate records—beyond that,
the representation is less precise. This accuracy
range is represented by a scale of grays either side
of the mid-tone, from dark tones with detail (for
example, dark hair with some individual strands
distinguish able) to light tones showing texture (for
example, paper showing creases and wrinkles, and
perhaps some fibers). If you locate your exposure
so that the most important tone of your image
What is “correct”?
To the eye, this scene
was brighter and less
colorful than seen here.
A technically correct
exposure would have
produced a lighter
image—thus failing to
record the sunset colors.
Although it looks like this
might challenge a basic
system, by placing the sun
center frame you guarantee
underexposure, resulting,
as in this example, in a
visually better image
than a “correct” exposure
would produce.

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