Some photographers prefer to judge the quality of an image's exposure by viewing its histogram rather than the actual photo in the LCD screen. So, let's take a look at this thing they call the histogram. If you're very technically inclined and enjoy mathematics and charts, you'll probably enjoy histograms. Also, if you're new to exposure issues and find yourself continually frustrated with the task of identifying good vs. bad exposures, you might learn a lot by looking at your histogram.

A histogram is a graphic representation of all of the pixels in a digital photograph. Think of it as a simple bar chart, like one you might find in PowerPoint or accounting software. Across the horizontal axis, you see a spectrum representing all of the colors available to each photograph, from lightest to darkest. Along the vertical axis, you are presented with the amount of color in your photoi.e., how many pixels consist of each particular tone.

Bright images have a lot of high peaks on the right end of the histogram. This usually indicates a tonal imbalance, with the photo leaning toward the brighter tones. Dark images have a majority of peaks on the left end. This usually indicates that the image is too dark. The middle-of-the-road version, wouldn't you know it, has the most peaks in the middle of the histogram.

Generally, then, the thing you want most in the histogram is to have the majority of pixels in the middle, with this middle "mountain" of black stretching across the horizontal axis as far as possible. Too much on the left and too much on the right often means under- or overexposure.