Levels

Choose Image > Adjust > Levels to enhance the tonal range of the image.

To move the black point

If the black point is too bright (far off the left edge of the histogram), shadows are gray not dark. To correct this you have to move the black point. Choose Image > Adjust > Levels and drag "Black" slider to the right.

To move the white point

If the white point is too dark (far off the right edge of the histogram), highlights are dark not bright. To correct this you have to move the white point. Choose Image > Adjust > Levels and drag "White" slider to the left.