so lets say you see a nice scenery, you take a picture that is -2.0 (under exposed), 0.0 (normal exposure), and +2.0 (over exposed), you can use photoshop or other third party apps that specialize in combining images (i recommend trying out Photomatix) so that you end up with a picture that has all the values of 2.0, 0.0 and +2.0.