Jean, I know there is a way that you can use Photoshop to select in your image a lightest light and a darkest dark, and then re-calibrate the rest of the image to fit inside those benchmarks. I think this would be a useful manipulation to be able to make.
I came across this while browsing through
"50 Fast Photoshop 7 Techniques," by Gregory Georges, at the local bookstore. I'll be ordering it through the Stroke of Genius link.