1. A method for selectively enhancing a portion of a digital halftone image in a system having a screen function with a variable parameter, the parameter capable of being changed to continuously or discretely vary the contrast of the screen for image enhancement, said image including a plurality of pixel elements, comprising the steps of: (a) scanning a portion of the image to determine the maximum and minimum grey scale levels within said portion, (b) determining the difference between the maximum and minimum grey scale levels within said portion, and (c) changing the parameter to selectively vary the contrast of the screen in response to the difference between the maximum and minimum grey scale levels for enhancement of said portion of the digital halftone image.