A) The slope of the transfer function: density (log of reciprocal transmission ) vs. log exposure. It is thus the power function correlating transmission to exposure. b) Gamma in the photographic sense was originally applied specifically to the straight-line portion of the transfer function. Only if all of the photographic densities corresponding to light intensities in the scene lie within that straight-line portion of the transfer function is gamma proportional to contrast. It is sometimes loosely used to indicate either an average or a point slope of the transfer function.

Note: In the photographic system, increasing gamma increases image contrast.