Originally Posted by MikeWhy
The purpose is to get two or more different views of the same subject. My understanding is that it's more than just a central occlusion: each of the paired AF sensors gets light from an aerial image of a virtual subaperture at opposite sides of the f/2.8 annular ring. If it were designed to see more of the center, the accuracy would decrease in proportion with the decrease in baseline. If it were designed to see more of the outer aperture (e.g. f/1.4 lens), then it wouldn't work at all with f/2 or f/2.8 lenses (just as f/2.8 sensors don't work at all with f/4 or f/5.6 lenses). It only sees just that part of the lens that it's designed to (within certain tolerances... f/5.6 AF sensors will sometimes see a little of the f/8 ring, which is why they can kinda-sorta autofocus, though with a smaller baseline.)