Know your curves
September 12, 2007
How do you read a histogram? Glad you asked!
A histogram is a graph of 256 vertical bars, one for each tonality in an 8-bit image. (For a 16-bit image it will show it reduced to 8-bit.) The left end represents black, and the right end, white.
The two ends are very important. In most cases you want them to tail off as shown here. You don't want them to "push up against the wall" unless the image has a lot of dark or light.
There is no right way for the middle to look. It is as variable as images are. (The three shown here are all for the same image at different contrasts.) For an average exposure image, shown here, the center of the "mountain peaks" will be near the center of the graph.
An average contrast image will have the vertical bars spread from darks to lights, as shown above. A lower contrast image will have them more in the center (below, left); A higher contrast image will have them distributed more widely (below, right.)
While the example here is in Adobe Photoshop's "Levels" controls, these principles can also be used for reading and evaluating histograms in your camera.
Diane Miller is a widely-exhibited freelance photographer who lives north of San Francisco, in the Wine Country, and specializes in fine-art nature photography. Her work, which can be found on her web site, www.DianeDMiller.com, has been published and exhibited throughout the Pacific Northwest. Many of her images are represented for stock by www.MonsoonImages.com.