Patent ID: 7826735

Claim:
An auto focus unit that brings an optical image of a target-object into focus on a light-receiving surface of an imaging device, the optical image of the target-object being included in an entire optical image, the entire optical image being incident onto the light receiving surface through a photographic optical system, the auto focus unit comprising: a pursuit block that pursues movement of an optical image of said target-object within an effective imaging area of the imaging device; a lens driver that causes a focus lens of the photographic optical system to move so that an optical image of a constant size scanning-area is focused on the light-receiving surface, a part of said optical image of said target-object being captured in said scanning-area; a detection block that detects a light intensity of an area including at least said scanning area; and a controller that causes said pursuit block to suspend pursuit of said target-object when said light intensity detected by said detection block is lower than a predetermined threshold value, and said predetermined threshold value is changed in accordance with a change in focal length of the photographic optical system; wherein a plurality of predetermined threshold values are stored and one of the stored threshold values is selected based on a focal length of the photographic optical system, and wherein pursuit is sustained or suspended in response to a result of a comparison between the predetermined threshold value and the detected brightness value.