"To (badly) paraphrase, sharpness tries to apply an edge to borders; but in doing so it can give a less than natural effect (often a halo) if set too high, it is "guessing" what the border should look like. If the source is good then Joe Kane believes sharpness should turned right down, I have to say that I agree with this. If the source is very "soft" (e.g. VHS), then an appropriate amount of sharpness can be set, without making the picture look false."