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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.

1 is pretty often "how much contrast", which is pretty easy to do.

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.

http://en.wikipedia.org/wiki/Autofocus

Passive: Measurement of light

Active: Infrared or ultrasound