Canon PowerShot S2 IS User Manual page 34

While a recorded image is displayed on the LCD monitor, you can
check the exposure with the graph (see Histogram Function (p. 32))
that shows the distribution of the brightness data. If the exposure
requires adjustment, set the exposure compensation (p. 90) and
reshoot. Press the DISP. button if the histogram and other information
are not shown.
Please note that image information recorded by this camera may not
display correctly on other cameras and image information recorded by
other cameras may not display correctly on this camera.
Histogram
The histogram is a graph that allows you to judge the brightness of the
shot image. The greater the bias toward the left in the graph, the darker
the image. The greater the bias toward the right, the brighter the
image.
If the image is too dark, adjust the exposure compensation to a positive
value. Similarly, adjust the exposure compensation to a negative value
if the image is too bright (p. 90).
Dark Image
Overexposure Warning
In the following cases, overexposed portions of the image blink.
When reviewing an image right after shooting in the LCD monitor
(information view) or in the viewfinder
In the detailed display mode of the Playback mode
32
Balanced Image Bright Image