Using the Auto Contrast command

The Auto Contrast command adjusts the overall contrast and mixture of colors in an RGB image automatically. Because it does not adjust channels individually, Auto Contrast does not introduce or remove color casts. It clips the shadow and highlight values in an image and then maps the remaining lightest and darkest pixels in the image to pure white (level 255) and pure black (level 0). This makes the highlights appear lighter and shadows appear darker.

By default, when identifying the lightest and darkest pixels in an image, Auto Contrast clips the white and black pixels by 0.5%--that is, it ignores the first 0.5% of either extreme. You can change this default using the Auto Color Correction Options found in the Levels and the Curves dialog boxes. For information changing the defaults in Auto Contrast, see Setting auto color correction options (Photoshop).

Auto Contrast can improve the appearance of many photographic or continuous-tone images. It does not improve flat-color images.

To use the Auto Contrast command:

1. Do one of the following:
• Choose Image > Adjustments > Auto Contrast. The application of Auto Contrast 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 the Enhance Monochromatic Contrast option 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 Contrast. For more information on adjusting the settings in the Auto Color Correction Options, see Setting auto color correction options (Photoshop).

To change the Auto Contrast 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.

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