Umut: D-max for a negative film is simply the maximum density (opacity) the developed metallic silver is capable of producing. A higher density means less light is transmitted. For papers it is the same thing (ie density of deposited silver) but is defined by reflection density since the print is viewed by reflected rather than transmitted light. So for negatives and transparencies, d-max results in the least amount of light transmission. For prints, d-max results in the least amount of light reflection ("maximum black").