All the replies have been interesting and informative but in what instances is it necessary or preferrable to use the masking process? I'm particularly interested in the masking for sharpness control. For example, the masking to improve sharpness is in what sense of the word? Can out of focus issues from loss of DoF be made more sharp or is to enhance the sense of accutance in the negative? When exactly would this type of masking be necessary to perform?