2. Create an adjustment layer Levels and add some brightness and contrast (movie).

Well, the girl to your left looks better now, while the girl to your right is too bright.

3. Activate the Gradient Tool*; then drag it on the image (animation).

*Make sure that: Linear Gradient selected on Options bar; foreground color is white; background color is black; layer Levels is selected in Layers palette.

We have just equalized tone by creating a gradual layer mask . It seems though that the entire image lacks contrast now.

4. Create another layer Levels (or Brightness/Contrast) and make the final tonal adjustments (animation).