A method of producing a tone scale function which can operate on a source digital image to improve tonal characteristics, includes generating the tone scale function having a highlight tone scale segment and a shadow tone scale segment defined relative to a reference point on the tone scale function, and that is adapted to operate on the source digital image to improve its tonal characteristics by: using a first function with a monotonically decreasing slope characteristic to produce the highlight tone scale segment that includes points that are equal to or greater than the reference point; and using a second function with a monotonically increasing slope characteristic to produce the shadow tone scale segment that includes points that are equal to or less than the reference point.