This method combines the best of the other Levels methods.
If you haven't already done so, go to Levels Introduction.
Be sure to check off as you go along.
Create a Levels adjustment layer, or reopen its window by double clicking the Levels window thumbnail in the Levels adjustment layer.
2) Click Reset if the changes are not acceptable.
If the color changes, and it's not acceptable, change the blending mode of the Levels adjustment layer from Normal to Luminosity.
Blending mode is located at the top of the Layers palette.

For more about the Auto button, go to the Auto Button Method.
There are three triangular sliders below the histogram of your photograph.
1) Press Alt, click the white slider, and hold.
The photograph will turn black.
Move the slider to the left until you see a white area appear.
If there's a white area present before you start to move the slider, ignore that area.
2) Press Alt, click the black slider, and hold.
The photograph will turn white.
Move the slider to the left until you see a black area appear.
If there's a black area present before you start to move the slider, ignore that area.
Contrast is the difference between the shadows and highlights.
1) Click the gray slider, in the middle, and hold.
Don't press and hold Alt, like you did for step two above.
2) To increase the contrast, move the slider to the right.
3) To decrease the contrast, move the slider to the left.
For more about using all three of the sliders, go to the Sliders Method.
1) Click the middle eyedropper, the gray-point eyedropper.
2) Look for a gray area on your photograph.
The gray area may not be gray, but should be.
It isn't gray because there's a color caste.
4) Click Reset if the color correction isn't acceptable.
For more about this eyedropper, go to the Gray-point Eyedropper Method