An instrument for determining the sensitivity of a photographic film to light. The film is given either a stepped exposure or a continuously varied exposure; after processing, the density is measured with a densitometer and the H and D curve is plotted. From this, the film speed, gamma, etc., can be determined. Sensitometers are of two types: those in which the illumination is varied with a constant exposure time, and those in which the time is varied with a constant illumination. Most films show some degree of reciprocity failure for exposures at extremely short or extremely long exposure times, even though the product of time and illumination is held constant.