A film characteristic curve, also known as the H&D curve (after its originators Hurter and Driffield) or the D-log E curve, is a graphical representation that describes the relationship between the amount of light exposure a film receives and the resulting density of the developed image. The horizontal axis plots the logarithm of the exposure value, while the vertical axis plots the density - the degree of darkening - produced in the film emulsion after development.
The curve typically consists of three distinct regions: the toe, where shadow detail is recorded at low exposure levels; the straight line portion, where exposure and density have a more linear relationship and mid-tones are captured; and the shoulder, where the curve flattens out as the film approaches maximum density and highlight detail begins to be lost. Understanding these regions helps photographers predict how a film will respond across the full range of tones in a scene.
Each brand and type of film exhibits its own unique characteristic curve, which is determined by the chemical composition of its emulsion and the development process used. These differences influence a film's contrast, tonal range, and overall response to light, making the characteristic curve a valuable tool for photographers, darkroom technicians, and manufacturers when comparing and selecting film stocks for specific applications.