[miro]

I have my misgivings too, but Imaging Resource expects on-chip PD AF to be very accurate. The basis for their reasoning http://www.imaging-resource.com/PRODS/ca...n-70dA.HTM is outlined as follows:

 

"In any phase-detect AF system, focus accuracy is a strong function of the "baseline" of the AF sensor. The greater the angle of the incoming light rays and the longer the distance between AF sensor elements, the more precise the focus will be. This is why many higher-end cameras have

special "high accuracy" central points, that kick in at apertures of f/2.8 or larger. Taking advantage of the wider light cone at very wide apertures, they use a second set of AF pixels spread further apart from each other to achieve greater accuracy.

 

With the Canon 70D's image sensor, the effective AF baseline can be made as wide as the lens will

allow, since there's no restriction caused by a fixed AF-sensor geometry. As the aperture gets wider (and you really need the added focus precision due to shallower depth of field), the camera can simply

select a more widely-separated group of pixels to use for its focus measurement. There's no limit to this, either. An f/2 lens will give higher AF accuracy than an f/2.8 one, and an f/1.2 lens will be even

more so. Combine this with the guaranteed alignment between the "focus sensor" and the image sensor, and we should see unprecedented levels of focus accuracy with very wide-aperture lenses."

 

But, as they say, the proof is in the pudding.

Well,

the base distance in PD is determened by prism angle. Yes the different prism angle in PD requires different AF sensor but this has never been an issue for middle and pro range SLR. I still puzeling what is the advantafe of such system.

The only one that I can see is lowering the cost and reduce machanical requirements.