During image formation and acquisition, many types of distortions limit the quality of images. The question arises if, and if yes, to which extent the effects of the degradation can be reverted. It is obvious, of course, that information that is no longer present at all in the degraded image cannot be retrieved. To make this point clear, let us assume the extreme case that only the mean gray value of an image is retained. Then, it will not be possible by any means to reconstruct its content.