1. A method of processing a digital image of a document to provide a digital copy having a selected image quality, the digital image comprising an array of pixels representing at least a portion of said document, comprising the steps of: deriving from said digital image at least three bitonal copies thereof at corresponding distinct contrast sensitivities; providing a numerical measure of the change between each pair of said bitonal copies corresponding to adjacent contrast sensitivities when said corresponding distinct contrast sensitivities are arranged to form a monotonic sequence, whereby the collection of said numerical measures defines a numerical image-variation sequence representing variations in said digital image as the contrast sensitivity thereof is monotonically varied; deriving from said numerical image-variation sequence and said corresponding distinct contrast sensitivities a first optimizing contrast sensitivity; and providing a bitonal digital copy of said digital image at said first optimizing contrast sensitivity.