Changes in viewing conditions for original scenes and hard copies cause the brightness function of human eye to change. When one assumes that the target of tone reproduction in hardcopy is relative brightness reproduction, the HD curve for a hardcopy system must be such as to compensate that kind of change in brightness function. According to Bartleson's treatment, derivations of desirable tone reproduction curves for photographic films will be described together with some expansions to the cases of CG recording.