Cheat Sheet: Histogram Cracker

We at Pop Photo constantly admonish you to “check your histogram!” (“Nature,” November 2003). Digital gurus always emphasize the importance of using the histogram to gauge proper exposure when shooting, and advise going to the histogram first to make corrections. But even technophile photographers who used to don goggles and gloves to mix their own chemicals can be baffled by the apparent scientific impenetrability of the histogram. But once you get familiar with its nuances, it’s not such a big deal.

A histogram is a chart, a visual representation, of the commonness of each tone in an image. The more often a tone occurs, the bigger its lump on the chart. Generally, the tones read from darkest black on the left to lightest white on the right. So if you’ve got an underexposed or low-key image, you see big bumps on the left; if you’ve got an overexposed or high-key image, you see big bumps on the right.

Because every image is different, every histogram is different—there’s no perfect template. A properly exposed image rife with midtones and containing a moderate amount of highlights and shadows would feature humps distributed across the scale of dark to light (see example above). While there’s no ideal histogram, there is such a thing as a bad one. You never want to see tall bars crunched up at either edge. That means the highlights or shadows are getting clipped and detail lost. If you see one of those when you’re shooting, correct your exposure to include all the tones you want in the scene. A good histogram will always show the whole mountain, no matter how far to the left or the right it peaks. Add exposure (plus compensation) if the histogram is scrunches up on the left; subtract exposure (minus compensation) if the graph is scrunched on the right.