Graphics Programs Reference
In-Depth Information
The next option on our list of adjustment layers is called
Levels
. This adjustment is fast
and effective. It will dramatically improve most images. Basically, it alters the effective
range of tones in the histogram.
It is like discarding the highs and lows on a bell curve and redistributing what remains.
The
Levels Adjustment Layer
gives us the ability
to alter the histogram in a way that adds contrast
and impact to our image. The left side of a histo-
gram represents pure black and the right side rep-
resents pure white. Everything in between is a
shade of gray.
A
Photographic Histogram
is composed of 256
vertical bars. The height of each bar is determined
by the relative amount of image data that has that
particular tonal value. For example, the height of
vertical bar 156 is determined by how many pixels
in the image have the exact same tone (shade of
gray in a Black & White Image) as that position
between pure black and pure white. Bar 156 is
156/256 or 61% of the way from black to white.
Notice how the histogram in the last image tappers off on the left side. This means that
there is very little image data beyond a certain value on the left. If we discard that data and
redistribute the pixels that remain, the image will be more realistic and have more impact.
