1 Click Options in the Levels dialog box or Curves dialog box.
2 Specify the algorithm you want Photoshop to use to adjust the overall tonal range of an image:
• Enhance Monochromatic Contrast clips all channels identically. This preserves the overall color relationship while making highlights appear lighter and shadows appear darker. The Auto Contrast command uses this algorithm.
• Enhance Per Channel Contrast maximizes the tonal range in each channel to produce a more dramatic correction. Because each channel is adjusted individually, Enhance Per Channel Contrast may remove or introduce color casts. The Auto Levels command uses this algorithm.
• Find Dark & Light Colors finds the average lightest and darkest pixels in an image and uses them to maximize contrast while minimizing clipping. The Auto Color command uses this algorithm.
3 Select Snap Neutral Midtones if you want Photoshop to find an average nearly neutral color in an image and then adjust the gamma values to make the color neutral. The Auto Color command uses this algorithm.
4 To specify how much to clip black and white pixels, enter percentages in the Clip text boxes. A value between 0.5% and 1% is recommended.
By default, Photoshop clips the black and white pixels by 0.5%—that is, it ignores the first 0.5% of either extreme when identifying the lightest and darkest pixels in the image. This ensures that white and black values are based on representative rather than extreme pixel values.
5 To assign color values to the darkest, neutral, and lightest areas of an image, click a color swatch. For guidelines on setting color values, see "Using target values to set highlights and shadows (Photoshop)" on page 141.
6 Do one of the following:
• To save the settings for use in the current Levels or Curves dialog box, click OK. If you subsequently click the Auto button, Photoshop will reapply the same settings to the image.
• To save the settings as the default, select Save as Defaults, then click OK. The next time you open the Levels or Curves dialog box, you can apply the same setting by clicking the Auto button. The default clipping percentages are also used by the Auto Level, Auto Contrast, and Auto Color commands.
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