In the previous section we adjusted the black/white points using sliders in Levels to restore dynamic range in an image. Since this is a popular technique, Photoshop includes commands that automate this action. Under Image > Adjustments you will find Auto Contrast, Auto Levels, and Auto Color.
Alternatively, you can execute these commands from Levels or Curves with the Auto button. To see which Auto command executes, click the Options button.
The three methods are listed below, with their equivalent selection in the Options dialog.
Auto Contrast. Enhance Monochromatic Contrast.
Same as choosing the RGB combined channel in Levels and moving the black/white input sliders toward middle. Expands contrast for all channels equally. Improves contrast without altering color balance.Auto Levels. Enhance Per Channel Contrast.
Same as moving the black/white input sliders in Levels toward the middle for each channel individually. Improves contrast and removes color cast in images that have at least a few pure black and white areas.Auto Color. Find Dark and Light Colors.
Similar to Auto Levels, but works well with images that do not contain pure black/white areas.
If you enable the Save as Defaults checkbox, any settings you make become the new default, and will be applied when you click the Auto button in Levels or Curves.
Occasionally you'll encounter spikes at the endpoints of an image that will prevent the Auto commands from working properly. The following was obtained from an infrared image. In this case the spikes contained no useful information so they were truncated in Levels as shown.
Although you can set defaults for Auto commands, each image has its own requirements. One tactic that works well is to avoid the Auto button in the Levels and Curves dialog box and go straight to Options. There you can easily choose among the three different Auto commands.
See the section on Black/White Points for an explanation of target colors and clipping.
