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In working with histograms, your goal should be to have all the tones in an image spread
out between the edges, with none clipped off at the left and right sides. Underexposing
(to preserve highlights) should be done only as a last resort, because retrieving the under-
exposed shadows in your image editor will frequently increase the noise, even if you’re
working with RAW files. A better course of action is to expose for the highlights, but,
when the subject matter makes it practical, fill in the shadows with additional light,
using reflectors, fill flash, or other techniques rather than allowing them to be seriously
underexposed.
The more you work with histograms, the more useful they become. One of the first
things that histogram veterans notice is that it’s possible to overexpose one channel even
if the overall exposure appears to be correct. For example, flower photographers soon
discover that it’s really, really difficult to get a good picture of a rose, like the one shown
in Figure 4.23. The exposure looks okay—but there’s no detail in the rose’s petals.
Looking at the histogram (see Figure 4.24) shows why: the red channel is blown out. If
you look at the red histogram, there’s a peak at the right edge that indicates that high-
light information has been lost. In fact, the green channel has been blown, too, and so
the green parts of the flower also lack detail. Only the blue channel’s histogram is entirely
contained within the boundaries of the chart, and, on first glance, the white luminance
histogram at top of the column of graphs seems fairly normal.
Any of the primary channels, red, green, or blue, can blow out all by themselves,
although bright reds seem to be the most common problem area. More difficult to diag-
nose are overexposed tones in one of the “in-between” hues on the color wheel.
Chapter 4 ■ Getting the Right Exposure 131
Figure 4.23
It’s common to
lose detail in
bright red flow-
ers because the
red channel
becomes over-
exposed even
when the other
channels are
properly
exposed.