Apple Aperture 2 - Manual (Page 361)

Chapter 14 An Overview of Image Adjustments 361
III
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.
Evaluating Tonality and Contrast
Although histogram graphs are good tools for evaluating an image’s exposure, you
shouldn’t interpret histograms for exposure information only, because the shape of the
histogram is also influenced by the tonality in the scene. You need to take the subject of
the image into account when evaluating its histogram. For example, images shot at night
are naturally going to have a majority of peaks in the darker side of the histogram.