When working with the Levels adjustment layer, you may have noticed the Auto button. This command button triggers an analysis of the histogram data by Photoshop that is then used to modify the individual controls of the Levels adjustment. In many cases this results in an image that is properly adjusted for color balance and exposure issues. In others it will get you closer to a corrected image.
Step 1. Open the file Auto_Levels.jpg (image provided for educational use only).
Step 2. Add a Levels adjustment layer by clicking the Levels icon in the Adjustments panel.
Step 3. Click the Auto button to perform an automated adjustment for the image. The images levels and color are adjusted.
Step 4. To refine how the automatic adjustment works, hold down the Option (Alt) key and click the Auto button again. A new dialog box opens.
Step 5. Select Enhance Per Channel Contrast and Snap Neutral Midtones to create a very natural balance of colors for the image.
Step 6. Click OK to close the dialog box.
Step 7. Adjust the middle slider (gray) to refine the levels adjustment to taste.