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.
A histogram can also be used as a tool to evaluate whether there’s 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 one another without affecting the other areas of the image.
For information about performing a Levels adjustment, see Adjust the levels of an 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 one another without affecting the other areas of the image.
For information about performing a Curves adjustment, see Adjust the tonal curve of an image.