Contrast-based autofocus mechanism

Abstract

A contrast autofocus mechanism for a still camera automatically controls the displacement of an imaging lens (10) so as to cause an image of a subject (32) to be quickly brought into focus upon the len's image-receiving plane (24) in response to successive contrast measurements taken at different conditions which produce different depths of field. The change in depth of field may be accomplished by changing the aperture setting for successive contrast measurements. Based upon difference in contrast for the two measurements, the extent to which the lens is out-of-focus is calculated. This calculation is used to define the displacement of the lens to a new position which should be at or nearly at a position of peak contrast. If at the new position further refinement is necessary, a trial and error subroutine is conducted to determine in which direction the lens should be moved to attain focus. In addition, during this contrast measurement, the amount by which the lens is to be displaced in the correct direction is calculated and the lens is displaced to what should be an in-focus condition. To accommodate low contrast scenes an auxiliary default position measurement sequence may be carried out to determine when focus has been achieved or is incapable of being achieved. For each set of contrast measurements a blur circle (b') difference value, representative of the difference in contrast, is compared with thresholds to determine in which direction the lens should be moved.