Contrast is the difference between light and dark. Subtle control over tonal contrast is one of the most difficult things for an artist to master. Mastery of it however creates the illusion of light in a picture, and gives the artist the ability to transcend the surface appearance of the paint. To manipulate the affects of light it is necessary to give attention first to the darks. As paint has a smaller contrast range than real life it is necessary to sacrifice some areas and simplify and compress the tonal range used. The photographers old principal of a dark, a light and a full range of tones between is an excellent concept to consider.