Understanding How to Read Histograms

The histogram is a graph that displays relative brightness in an image, from pure black to pure white. The area under the graph represents all the pixels in the image. From left to right, the histogram describes the range of dark pixels (shadows), gray pixels (midtones), and bright pixels (highlights) in the image. The shape of the histogram graph depends on the tonality of the scene and the exposure.

Figure. Illustration showing how a histogram graphs a pixel's luminosity according to its tonal position.

A histogram can also be used as a tool to evaluate whether or not there is enough shadow, midtone, and highlight information in the image. Aperture provides three histograms in the Adjustments inspector and the Adjustments pane of the Inspector HUD. The histogram above the adjustment controls indicates the current state of the image. The Levels histogram included with the Levels adjustment controls provides a way to adjust the brightness values in the image in relation to the displayed histogram. You use the Levels controls to adjust the shadow, dark quarter-tone, midtone, light quarter-tone, and highlight values independently of each other without affecting the other areas of the image. The Curves histogram included with the Curves adjustment controls provides a way to adjust the tonal values in the image in relation to the displayed histogram. You use the Curves controls to adjust the full range of tonal values independently of each other without affecting the other areas of the image.

For more information about performing Levels adjustment, see Working with the Levels Controls. For more information about performing Curves adjustment, see Working with the Curves Controls.

Evaluating Exposure

Histograms are good tools for evaluating exposure. For example, a series of peaks in the darker side of the histogram often indicates an underexposed image that consists mainly of dark pixels.

Figure. Side-by-side comparison of an underexposed image and its histogram, with the peaks concentrated in the left side of the graph.

A series of peaks in the center of the histogram often indicates a balanced exposure because a majority of the pixels are concentrated within the midtones of the histogram. They’re not too dark or too bright.

Figure. Side-by-side comparison of a correctly exposed image and its histogram, with the peaks concentrated in the center of the graph.

However, a series of peaks in the brighter side of the histogram often indicates an overexposed image because most of the pixels in the image are too bright.

Figure. Side-by-side comparison of an overexposed image and its histogram, with the peaks concentrated in the right side of the graph.

Evaluating Tonality and Contrast

Although histogram graphs are good tools for evaluating an image’s exposure, you shouldn’t interpret histograms for exposure information only, because the shape of the histogram is also influenced by the tonality in the scene. You need to take the subject of the image into account when evaluating its histogram. For example, images shot at night are naturally going to have a majority of peaks in the darker side of the histogram.

Figure. Side-by-side comparison of an image shot at night and its histogram, with the peaks concentrated close to the left side of the graph.

Likewise, images of bright scenes, such as snow or light reflecting off the ocean, have a majority of their peaks in the brighter side of the histogram.

Figure. Side-by-side comparison of a bright image and its histogram, with the peaks concentrated in the right side of the graph.

Histograms can also depict contrast in an image. For example, this silhouette of the man in the hammock in front of the sunset consists of a relatively even assortment of extreme bright and dark tonal values with few midtones. In this case, the histogram is shaped like a valley with peaks in both the dark and bright sides.

Figure. Side-by-side comparison of an image shot in silhouette and its histogram, with the peaks concentrated in both the left and right sides of the graph and no activity present in the middle.

Likewise, histograms can also depict a lack of contrast in an image. For example, an image of a rainbow in the fog lacks contrast. Without directional lighting, there aren’t any highlights or shadows in the image. In this case, the peaks of the histogram are concentrated in the center and do not come close to either the dark or bright sides.

Figure. Side-by-side comparison of a low-contrast image and its histogram, with the peaks concentrated in the center of the graph and no activity present on the left or right side.