While reading through the manual today I found it odd that the 24-70mm f/2.8 is classified as group B which gives it only a single dual cross-type AF point.
Can anyone explain why this and other f/2.8 lenses are put into group B and even group C (no dual cross-type)?
It has to do with the exit pupil location of these lenses.
AF sensors are all about angles, not the quantity of light. F/5.6 sensitive AF sensors have a 10 degree offset (two sensors "looking" at opposite edges of the lens circle from the back) and f/2.8 sensitive AF sensors have a 20 degree offset. With lenses such as macro lenses, the exit pupil moves farther away as it focuses close, putting the edge of the circle outside the view of the AF sensors. The sensor is "blind" to the image, so the camera has to switch to an f/4 or f/5.6 AF sensor.
Same thing with the 24-70, which has an unconventional backward zoom, with 24mm extending the lens and 70mm making it shorter. At least, that's my hypothesis with this lens.
I suspect the 24-70 Mk II will be able to utilize these double cross points.
The great news with this sensor is all of the f/4 sensitive cross points. I think this AF unit is going to be one of the best ever marketed by anybody. The D800's sensor is nothing but f/5.6 AF points, other than that single f/8 center point.