The Modulation Transfer Function is based on Fourier techniques which imply the validity of the principle of superposition. This is clearly violated in the nonlinear film development process. It becomes necessary to face up to this question if one is to know how much confidence to place in the results of such analysis. A pseudotransfer function is therefore defined to represent an artificial linear filter which, if it replaced the actual photographic process, would result in precisely the same image as would be produced by the real system. The results of a computer program are presented to illustrate how such an approach may be used. Some of the conditions under which the atmospheric transfer function remains undefined are also reviewed.