Submitted by Al …
(Adapted from The Encyclopedia of Photography ©1963)
The characteristic curve is the graphic expression of an emulsion’s density-exposure relationship. Sometimes called the sensitometric or D-log-E curve, it was developed as a result of the work of Hurter and Driffield in the field of sensitometry. It is also often referred to as the H&D curve.
In order to calculate the curve, the film or paper emulsion is given regulation-stepped exposures which are calculated with strict accuracy, then it is developed in a standard formula according to uniform development methods. The densities, measured by a densitometer, are represented in the height of the curve, and the logarithm of the exposures are on the base.
The result is a curve (of varying shapes, according to the emulsion) having a ‘foot’ or base, a middle straight-line portion, and a ‘shoulder’ or sloping-off portion at the top. The straight-line portion is the area of correct exposure, where each increase in exposure causes a similar increase in density. The foot represents the area of underexposure, while the shoulder represents the area of overexposure.
Characteristic curves are used in determining the latitude and the contrast of emulsion. From these curves, gamma (or development factor) can also be calculated.
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