Guidelines for Analyzing the Histogram

Here are guidelines that can help you analyze the histogram and make decisions about how to correct your images:

If a line spikes, there are many pixels at that value.
If a line is close to the horizontal axis, there are few pixels at that level.
If the graph is spread out, the image has a balanced composition and probably has enough detail to be corrected.
If the lines are compressed into a narrow area, the image probably doesn’t contain enough detail to be corrected; try a new scan or find another picture.
The usefulness of the histogram depends upon the image content as well as its channel values. For example, an image of pale, desaturated skin tones may display a big saturation peak on the left side (in the lower values) of the histogram. An image of a bright red fire engine may display a peak on the right side (in the higher values) of the histogram. If the images were improperly saturated, the saturation histograms might be reversed.
If the greyscale graph is mostly at the left side, the image is too dark.
If the greyscale graph is mostly at the right side, the image is too light.
If the greyscale lines are not spread out enough, you may need to increase the contrast.