1. Algorithm to evaluate how blurred an image is. 2. Adjust the focus a little bit, is it getting less blurred? Keep going. More blurred? Other direction. 3. Keep adjusting until you find the point where adjusting either way means more blur.

its a passive system where the camera sensor collects image data before the picture is taken and analyzes the image, then activates the focus motor to adjust the focus. No emitter required.