Choose Layers > New Adjustment Layer >
Levels, or open an existing Levels adjustment layer.
Choose RGB from the Channel menu. When set to RGB, your adjustment
affects all three (red, green, and blue) channels. If you are working
on a grayscale image, it will only have the gray channel.
Set the shadow and highlight values by dragging the black
and white Input Levels sliders (the left and right sliders directly
under the histogram) to the edge of the first group of pixels on
either end of the histogram. You can also enter values directly
into the first and third Input Levels text boxes.
Dragging the highlight slider to the left lightens the photo.
Option-drag the Shadow slider to see
which areas will be clipped to black (level 0). Option-drag the
Highlight slider to see which areas will be clipped to white (level
255). Colored areas show clipping in individual channels.
To adjust the brightness of the middle tones without
affecting the shadow and highlight values, drag the gray Input Levels
(middle) slider. You can also enter values directly in the middle
Input Levels text box. (A value of 1.0 represents the current unadjusted
midtone value.) Click OK.
You can see the adjustment reflected in the Histogram palette.
Note: You can click Auto to move the Highlight and Shadow sliders
automatically to the brightest and darkest points in each channel.
This is the same as using the Auto Levels command and may cause
a color shift in your image.