The image on the right shows how using the Find Edges filter from Photoshop accentuates the edges of the scanned photo. I only use this filter when no amount of contrast editing helps me to see the lines of the image that I need to trace over with Illustrator. Don't let the "wierdness" of the image at right throw you. The reason for using this filter, or even the levels or contrast windows, is not to make your scan look good. The purpose is to help you recognize where the lines need to be drawn after importing the image into your drawing program.