1. Open your image.
2. Convert to RGB if it is not already in that color mode.
3. Open the Levels function (Cmd-L)[CTRL+L].
4. Switch to the Red channel by using either the popup menu or key commands (Cmd-1)[CTRL+1].
5. Using the sliders, crop the histogram to redistribute image information (crop histogram tails).
6. Repeat step 5 for the Green (Cmd-2)[CTRL+2] and Blue (Cmd-3)[CTRL+3] channels.
7. Return to the composite RGB Levels, and observe the effect of the changes by looking at the image and make midtone adjustments accordingly.
8. Close the Levels by clicking the [ok] button.