This simple example shows that normal sharpening creates cleaner edges than the original. Over sharpening makes the circle look artificially sharp. This is achieved by creating a white external halo (making the light gray of the background brighter around the circle's edge) and an internal black halo (making the darker gray of the circle darker around the circle's edge). Because the difference between the white and black halos is larger than between the gray of the circle and the background, the edge contrast has been increased, creating the illusion of enhanced sharpness. But the halos are undesirable in photographic images and are extremely hard to undo, unless you shoot in RAW (see below).