Nikon D300 User Manual page 673

If you master this method, you will have a very fine degree of
control over where you place your image's light ranges. This
is sort of like using the famous Ansel Adams's black and
white Zone System, but it is represented visually on the
Monitor of your D300(S).
The manipulation of the histogram levels in-computer is a
detailed study in itself. It's part of having a digital darkroom.
Learn to use your computer to tweak your images, and you'll
be able to produce superior results most of the time. Even
more importantly, learn to use your histogram to capture a
nice image in the first place!
Your histogram is simply a graph that lets you see at a glance
how well your image is contained by your camera. Too far
left and the image is too dark; too far right and the image is
too light. Learn to use the histogram well and your images are
bound to improve!
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