read an interesting patent development the other week:
apparently they are starting to dedicate certain lines of the sensor for AF sensing, - i.e. multiple lines spread across the entire chip are used for Autofocus rather than image information.
the lines are then encoded out of the final image in the same way that bad lines are written out and masked (by duplicating a line above/below) in sensor testing.
(most MF sensors have faulty pixels/ lines deleted and correction 'masks' burned into their circuitry)
this bodes well for future developments:
- Live View with AF, where the mirror can be up and AF remains active (or better: deleted the mirror altogether, and go with electronic viewfinder)
- AF across the entire chip area from edge to edge, not just in the center. no single point/ multi point/ 51 point - but the entire full frame could have AF
it was a Nikon patent I believe, though I'm sure others can apply similar answers to MF chips.