In this lesson: Learn how to digitally view and manage image contrast and brightness

Studying the histogram allows you to view and balance the tonal values in your images, resulting in photos that have improved contrast and brightness.

The Histogram, Defined

A histogram, the colored graph seen in the sample images, is a graphic representation of the data gathered by the camera sensor when an image is taken. The data is divided into 256 levels of brightness and can be represented separately in the RGB color channels or viewed as overall luminosity values. 0, on the left side of the horizontal axis, represents black, while 128, in the center, represents middle gray, and 255, on the far right represents maximum white. The number of pixels that fall within any of the tonal ranges on the scale is represented by the height of a bar on the vertical axis.

Figure 1

Figure 1 shows the histogram created by Adobe Camera Raw when this image file was opened. Note the red, green and blue areas, representing the RGB values and the light gray area at the bottom, showing the averaged values of all three channels, representing overall luminosity. This histogram represents a relatively normally exposed image, with lower overall luminosity in the lower mid-tone range. Note how the extreme left and right ends of the graph show no sharp spikes.

Figure 2

In Figure 2, note how the histograms have shifted to the right and left, and the spikes at the respective ends of the graphs. These spikes at the extreme end indicate “clipping,” or loss of detail in the highlighted areas. Notice also the corresponding differences in the image from the first one – Figure 2 has brighter highlights in the clouds and is a brighter photo.

Figure 3

In Figure 3, we again see the shifted histogram with spikes on the left and right sides. In this instance, the image’s darker areas have been clipped, as is seen in the loss of detail in the darker areas of the shot. Note also that the low spot in the lower mid-tone range remains relatively constant in the images. This would indicate that it might be effective to adjust the exposure to bring out more detail in these areas. It may also simply be an indication that there are fewer areas in the scene within that tonal range.

The Advantages of Using the Histogram

Bear in mind that this demonstration uses the histogram available in post-processing software. A more effective method is to utilize the histogram built into your DSLR to evaluate your results when shooting. This is generally more accurate than observing your LCD display preview, which can be misleading. It’s also important to realize that there is no “correct” or “perfect” histogram. The graph should be used to achieve the results you want in an image. By learning to read camera histograms in the field or studio and adjust exposure settings accordingly, you can avoid blown out highlights or muddy shadow areas and similar issues, resulting in fewer ruined shots and shorter post-production times.

Challenge Activity

In today’s challenge activity, you will learn to activate and use your camera’s histogram, and rely on its display rather than the camera’s LCD to analyze the image based and adjust your exposure accordingly.

Step 1: Locate the histogram settings for your camera and activate it. Consult your camera’s owner’s manual for detailed instructions regarding how to do this, if need be.

Step 2: Photograph a high-key (bright) scene 3 times, exposing for the highlights, then the midtones, then the shadows. Note the resulting histograms for each.

Step 3: Repeat Step 2 with a low-key (dark) scene.

Step 4: Repeat Step 2 with an evenly-lit scene.

Step 5: Download all the images from the steps above and compare the histograms closely, noting how peaks and valleys, spikes, and blank areas correspond to changes in the images they represent. By repeating these exercises regularly, you’ll learn to use the histogram effectively to adjust your exposure in the field and make adjustments to white balance, contrast, and other factors in post production, to improve the overall quality of your images.