Novice photograpers often either don't know about the histogram or are too intimidated by it to try using it. The fact is, not only is it simple; once you've learned to use it, you'll find it's your best friend when it comes to judging exposure. There's nothing more disappointing than downloading a new batch of images and finding exposure problems that you couldn't see in the previews. The histogram can help you avoid that situation by letting you accurately assess the exposure of your photos at a glance.
Most DSLR cameras will let you turn the histogram on in the menu, then allow you to switch it on and off in the display when previewing your photos. Grab your instruction manual to find out how to access it. Once you've turned it on, take a few shots and familiarize yourself with viewing it. What you'll see is a graph. (If your camera shows you both a 3-color and a black version, select the black one for now.) It will form peaks and valleys with a series of bars, something like the one in the photo at the top of this article.
Of course, to get any use out of the graph, you'll need to know how to read it. Here's how it works: It separates all the pixels in your photo according to their luminosity, that is, how light or dark the pixels are. The darkest areas will display on the left side of the histogram and the lightest on the right. The height of each bar is determined by the total number of pixels in that luminosity range. Simple, right?
By examining the graph, you can judge the overall exposure of your photo and spot potential problems quickly, too. How? Like this: The peaks in the graph will show you which areas of brightness dominate the photo. If your photo is somewhat overexposed, the peak(s) will shift to the right side of the graph. Obviously, an underexposed image will show the higher values toward the left. If the left or right side of the graph are completely empty, it's likely that your photo is overexposed or underexposed.
Areas that appear to go "off the chart" will indicate clipping, meaning that the area of brightness is so dominant that others may be lacking in areas of the photo. For instance, a histogram with a very high peak on the far right edge indicates that the highlight areas of your photo probably have no shadow detail, in other words, they're "blown out". A sunset shot with that familiar semicircle of bright light with no detail would be a good example. It may not be possible to recover any detail in those areas later, so it's important to know about them so that you can adjust your exposure accordingly.
In the photo to the right, you can see that there are more dark tones than light, because the bird only takes up a small portion of the frame. That causes the main peak of the histogram to shift toward the left. You'll also notice small, but pronounced peaks at the far left and far right. Those indicate areas of fairly high contrast in the darkest shadow and brightest highlight areas. You can see some minor loss of detail in those areas of the photo.
High contrast areas will appear as portions of the graph that go sharply up and down, rather than forming a smooth curve. You should be seeing by now that you can assess your photo very quickly by a quick examination of the histogram, as well as see things that you may not notice in the preview. Now that you understand how to interpret the graph, you should have no trouble knowing whether you need to make adjustments to a shot.
If you'd like to see how the luminosity values are divided between the colors in your photo, simply switch to the RGB histogram if your camera gives you that option.
One last note: There's no such thing as a "perfect" histogram. The distribution of light in your photo is a matter of what you want from the shot. It's simply a guide that will help you see whether you've accomplished what you wanted to and an aid to adjusting your exposure when necessary.
The histogram is one of the most valuable tools in your photographer's toolbox. Learn to love it!