Well defined images span an entire range of color intensities.
However it is common to find images that do not. If a photo has
been overexposed it will be too bright - there will be few colors
at the low ends of intensity and many at the high end. Similarly if
a photograph has been underexposed it will be very dark - all the
colors will be at the low end of the range and virtually none at
the high end.
The Levels effect allows you fine control over brightness and
contrast to let you correct this kind of problem. The basic method
of adjustment is to set the black and white points on the input
image. Normally the black point will be at 0 and the white point at
255. This simply means that black is represented by the value 0 and
white is represented by the value 255.
However if an image is too dark there may be no pixels at all
with a value of 255. In this case what was white on the original
image might be represented by a value of only 200. By setting the
white input point to 200 and then applying the effect, the levels
in between will be stretched to try and restore balance to the
image. A similar operation setting the black input point would
apply if an image was too light.
As well as being able to specify input points you can also
specify output points. This lets you tell the effect what value
should be regarded as white and black on the final output
The levels effect is often used in conjunction with an image
histogram so that the exact representation of different color
levels can be seen in the image.