Nikon D300 User Manual page 660

Figure 9-14. A basic histogram
The histogram is basically a graph that represents the
maximum range of light values your camera can capture, in
256 steps (o = pure black, and 255 = pure white). In the
middle of the histogram are the mid-range values that
represent middle colors like grays, light browns, and greens.
The values from just above zero and just below 255 contain
detail.
The actual histogram graph looks like a mountain peak, or a
series of peaks, and the more there is of a particular color, the
taller the peak. In some cases the graph will be rounder on
top, and in other cases it will be flattened.
The left side of the histogram represents the maximum dark
values that your camera can record. The right side represents
the maximum brightness values your camera can capture. On
either end of the histogram the light values contain no detail.
They are either completely black or completely white.
The height of the histogram (top of mountain peaks)
represents the amount of individual colors. You cannot easily
control this value in-camera, other than changing to a Picture
Control with more or less saturated color, so it is for your
information only.
We are mostly concerned with the left- and right-side values
of the histogram, since we do have much greater control over
those (dark vs. light).
Simply put, the histogram's left and right directions are
related to the darkness and lightness of the image, while the
up and down directions of the histogram (valleys and peaks)
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