222 ABSOLUTE BEGINNER’S GUIDE TODIGITAL PHOTOGRAPHY
How Can Color Images Use Gray Values?
This talk of gray tones might seem foreign if you only work with color images. But
gray tones play a major role in color images, too. Remember that all color images
are made up of channels—plates that store information about a specific color. These
channels resemble the dye layers that make up color negative film: a layer of dye for
each color.
Most digital cameras use the RGB color space, which means there is a Red channel,
a Blue channel, and a Green channel. These channels by themselves are actually
black-and-white images that contain gray brightness values. As such, each channel
(R, G, and B) contains a combination of the 256 tonal values for brightness.Working with Histograms and Levels
Not all images share the same histogram. Here are some histograms of different
image types (see Figures 15.4–15.6) as they appear in the Levels dialog box:FIGURE 15.4
This histogram
shows a bal-
anced distribu-
tion of tones in
the image.
Chances are
good this image
is bright, con-
trasty, and
needs little, if
any, correction.FIGURE 15.5
This histogram,
with most of its
pixels at the
lower end of the
tonal range (in
other words,
dark pixels),
either needs cor-
rection or is a
night shot that
conveys a dark
mood.