In the first Levels example you made a Levels adjustment to all the channels evenly. In the Auto-Levels example, you let Photoshop adjust the levels and remove color cast using an automated algorithm. The Levels command can be further isolated to a specific channel by clicking the drop-down list in the center of the Levels dialog box. This allows you to tackle color cast issues, such as spill from a background, a bad white balance, or a photo shot under mixed or colored lighting.
Step 1. Open the file Levels_Color_Balance.jpg (provided for educational use only) photo by Jim Ball.
Step 2. Add a Levels adjustment layer using the Adjustments panel. You will use the Levels command to fix color and exposure issues.
Step 3. Select the Set White Point (white eyedropper) in the Levels dialog box. Click an area that should be pure white. For this image, click a bright area in the white pillar.
If you click an area that is not bright enough, the whites in the image will overexpose. (You can click the Reset buttonit looks like a circular arrowat the bottom of the Adjustments panel to reset the Levels command, if needed.) After you click, you’ll see that some of the color spill has been removed.
Step 4. Select the Set Black Point (black eyedropper) in the Levels dialog box. Click an area that should be pure black. Choose an area such as a jacket or a dark shadow. This will adjust the color balance and the exposure.
Step 5. The images color balance should now be better. Adjust the middle Input Levels slider to brighten the image.