Exposure, development and gamma
When a silver halide grain has been exposed to produce a latent image, the developer acts upon the grain to cause other molecules of silver halide to separate and form metallic silver. The small quantities of metallic silver on the surface of the grain act as a catalyst that allows the reduction of the silver halide by the developer.
This causes the exposed grains to be acted upon by the developer before the other unexposed grains. With prolonged development time the developer will act on unexposed grains causing an overall slight density increase, known as chemical fogging.
In simple terms, the more exposure, the more silver specks on the grain, the sooner the development will start. The longer the development the higher the contrast, up to a point. Chemical fogging and highlights developed to densities within the shoulder reduce the density difference as well as the inherent contrast of the film material. Figs 13.12i and 13.12ii show the results on the density difference of a scene with manipulation of the overall exposure and development.