Pentax K-7 Operating Manual page 31

A histogram shows the brightness
distribution of an image. The
horizontal axis represents brightness
(dark at the left and bright at the right)
and the vertical axis represents the
number of pixels.
<--Brightness--> (Bright)
Dark portions Bright portions
The shape and the distribution of the
histogram before and after shooting tells you whether the exposure level
and contrast are correct or not, and lets you decide if you need to use EV
compensation and take a picture again.
r-,_Adjusting the Exposure (p.115)
r-,_Adjusting the Brightness (p. 199)
Understanding Brightness
When the brightness is correct and there are no overly light or dark areas,
the graph peaks in the middle. If the image is too dark, the peak is on the
left side, and if it is too bright, the peak is on the right side.
Dark image Image with few bright Bright image
or dark areas
When the image is too dark, the part to the left is cut off (dark portions with
no detail) and when the image is too bright, the part to the right is cut off
(bright portions with no detail).
Bright portions blink red on the monitor and dark portions blink yellow
when [Bright/Dark area] is _ (On).
_,_Playing Back Images (p.78)
r-,_Setting the Display for Instant Review (p.263)