AUTO-FOCUS APPARATUS APPLIED TO IMAGE READING APPARATUS AND THE LIKE

In an auto-focus apparatus of the invention, an image sensor (e.g., a CCD) produces an image signal corresponding to an image on a microfilm. The image signal from the image sensor is encoded to a binary image signal by a binary encoding circuit. Alternation points (i.e., leading or trailing edges) of the binary image signal are counted by a counter. A system controller fetches count values from the counter, and detects an image position corresponding to a maximum count value. The system controller causes a lens driving circuit to move an imaging lens to the detected image position.