Depth of convergence of a sharpness index autofocus system

The critical parameters in the selection of an autofocus cost functional are that it must have a high sensitivity and selectivity at and near the correct focus point and that it must be capable of converging to the correct focus from a large initial or preclosed loop defocus state. The largest initial focal range error an autofocus system is capable of correcting in closed loop is referred to as the depth of convergence. It is shown theoretically and also verified experimentally that the depth of convergence is a sensitive function of the object range, the initial focal range of the imaging system, and, more remarkably, the spatial spectrum of the object.