What’s a Histogram?

A histogram is a graph that displays how light is distributed in your picture. The left side of the graph represents the shadows, while the highlights are on the right. This means that if the histogram has a high peak on the left, you can tell that a lot of pixels in the picture are dark, or in shadow. A peak on the right of the graph means that a lot of pixels are bright, or in highlights. Peaks in the middle of the graph represent pixels in the midtones of your exposure. Often it’s difficult to tell if your photograph is correctly exposed by looking at the image on the back of the camera particularly in bright light. I set my camera display so that I can see an image thumbnail as well as the histogram.
 

The Histogram of a Good Photo.
Let’s look at some examples of histograms. Refer to the picture below. On both the left side and the right side of the graph, you can see that there are no high peaks. This kind of histogram tells you that no part of the scene is over or underexposed. Ideally the graph information should fit within the left and the right hand side. 

The Histogram of an Underexposed Photo.
Below is a the histogram of an underexposed photo. You can see a spike in the shadows that starts with a peak on the left of the graph. That means that the picture has lost data in the shadows. There’s also just a few pixels trailing off the right side of the graph, so a tiny bit of data might have been lost there as well. Skin tones suffer particularly when an image is underexposed. To move the histogram over to the right and get a better exposure use the camera’s exposure compensation, and overexpose the photo until the graph fits within the left and right sides.
 

The Histogram of an Overexposed Photo.
Here you can see a pronounced spike on the right side of the graph. You’ll note that the height of the peaks is somewhat low in this picture; that’s not an indication of under or overexposure, it’s a measure of the spead of tones from absolute black on the left to absolute white on the right. All you need to worry about is whether they breach the left or right edges of the histogram.
 

 When composing your photo always try to avoid the histogram from spiking at either extreme end of the graph, where you’ll lose data and have under or overexposed parts of your picture. It’s important to make sure your image is correctly exposed at the time you take the photo as although you can fix minor over and underexposure problems by shooting in RAW, it’s often impossible to correct overexposed highlights.