the degree of distinctness in the border between two parts of a photographic image that have received different exposures. In general, the image boundary of an object having a brightness different from that of the surrounding background is always blurred. Depending on exposure conditions, the transition zone from high optical densities to low densities—the boundary region—is between 10 and 50 microns wide for modern photographic materials. A subjective impression of image definition depends on the rate at which the density in this zone changes and on the absolute difference between the densities at the zone’s edges.
Various methods of measuring definition have been proposed that use the maximum or root-mean-square gradient of the change in the optical density in the boundary region. The highest gradient—the best image definition—is achieved only at a certain optimum exposure. Unlike resolution, image definition characterizes the quality of reproduction for relatively coarse details of a photographic image. When the distance to an object is fixed, it is usually necessary to focus the lens in two different ways to obtain pictures with the best definition and resolution.
M. IA. SHUL’MAN