Using the Auto Levels command

The Auto Levels command automatically adjusts the black point and white point in an image. This clips a portion of the shadows and highlights in each channel and maps the lightest and darkest pixels in each color channel to pure white (level 255) and pure black (level 0). The intermediate pixel values are redistributed proportionately. As a result, using Auto Levels increases the contrast in an image because the pixel values are expanded (as opposed to being compressed, as in lower contrast images). Because Auto Levels adjusts each color channel individually, it may remove color or introduce color casts.

Auto Levels gives good results in certain images with an average distribution of pixel values that need a simple increase in contrast.

Note: By default, Auto Levels clips the white and black 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. The default settings for Auto Levels can be changed in the Auto Color Correction Options dialog box. For more information, see Setting auto color correction options (Photoshop).

To use the Auto Levels command:

1. Do one of the following:
• Choose Image > Adjustments > Auto Levels. The application of Auto Levels is automatic with this option. You cannot adjust any of the options in the following steps.
• (Photoshop) Choose Layer > New Adjustment Layer and choose either Levels or Curves. Click OK in the New Layer dialog box. Continue with the following steps.
2. In the Levels or Curves dialog box, click the Option button.
3. Select Enhance Per Channel Contrast under Algorithms in the Auto Color Correction Options dialog box.
4. Adjust the amount of shadow and highlight values that are clipped, and adjust the target color for the midtones.
5. Click OK in the two opened dialog boxes to apply Auto Levels. For more information on adjusting the settings in the Auto Color Correction Options, see Setting auto color correction options (Photoshop).

To change the Auto Levels defaults for target colors and clipping in ImageReady:

(ImageReady) Hold down Alt (Windows) or Option (Mac OS), and click Options in the Levels dialog box. Enter the percentage of extreme highlight and shadow pixels to ignore, and click OK. A value between 0.5% and 1% is recommended.