The histogram is a useful tool for determining and correcting exposure on both the camera and in your computer's image editor. The histogram is actually very easy to understand once you learn its basic components. The information on the left side of the graph represents the dark tones in your picture. On the right side you see a graphical representation of the highlight detail. The middle area represents the "gamma," or middle tones.
The Histogram on Your Camera's LCD Monitor
When you take a picture and review the image and its histogram on your camera's LCD monitor, you can accurately determine the exposure by looking at how the information on the histogram is distributed. If, for example, you're shooting a normal outdoor landscape with a broad tonal range, your histogram should have information throughout the graph from left to right. If the information is skewed to the left, however, then your picture is probably underexposed. If the information is tending to the right side of the graph, then your picture is most likely overexposed.
Using the Histogram in the Levels control panel can help you easily make exposure adjustments. Here's an uncorrected portrait. Notice the open gap on the right side of the graph information.
The above interpretation works for most shooting situations. But, if you're in very bright conditions or a dark environment, then you have to keep a couple things in mind. A bright scene, such as a skier on a snowy slope, should produce a histogram that is skewed to the right. That's because most of the information in the shot is bright. An evenly distributed histogram would actually produce a dull snowy scene. Same goes for dark interiors. They should be skewed to the left. Once you get the hang of matching histograms to shooting situations, they become a powerful ally in determining the proper exposure.
Using the Histogram in Photoshop
You can also use the histogram to make exposure adjustments in Photoshop (or your favorite image editor) after you've moved the images from your camera to your computer. Generally speaking, you can find the histogram in the "Levels" adjustment panel (Image > Adjustments > Levels).
Using the controls is straightforward. I usually start by moving the highlight pointer (right) to the right edge of the histogram, the shadow detail pointer (left) to the left side of the histogram, then adjusting the gamma (middle pointer) to my taste. This isn't a hard and fast approach, but it's a good starting point. Take a look at the adjusted image and how the histogram pointers have changed.
It's official. I've launched the Photo Assignment project. This month's theme is "Ice." You can read all of the details in this weblog post. We're going to have a lot of fun with Photo Assignment, so be sure to join in.
Listen to the Podcast
Now that I have your curiosity piqued, it's time to listen to today's audio show titled, "Learning the Histogram." You can download the podcast here (30 minutes).