Apple Aperture 3.5 - Manual (Page 345)

Chapter 7 Make image adjustments 345
About evaluating exposure
Histograms are good tools for evaluating exposure. For example, a series of peaks in the darker
side of the histogram often indicates an underexposed image that consists mainly of dark pixels.
A series of peaks in the center of the histogram often indicates a balanced exposure because a
majority of the pixels are concentrated within the midtones of the histogram. They’re not too
dark or too bright.
However, a series of peaks in the brighter side of the histogram often indicates an overexposed
image because most of the pixels in the image are too bright.
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