A major function of the photographic instrumentation engineer is to reduce phenomena to numbers, so he can measure, analyze, and manipulate. Photographic materials, e.g., films and papers, are part of his instrumentation. The purpose of this paper is to help the engineer, and others, understand that numbers can be assigned to characteristics of photographic materials, and what worth such numbers can have. The numbers discussed are those associated with sensitometry (speed, contrast, gamma, latitude) and especially those associated with image structure (RMS granularity, and modulation transfer function). Both color and black-and-white films are considered.