5D Mk III Lenses and Auto Focus Points Question

Page 79 of the new 5D Mk III manual begins describing how the AF points perform for each lens.
I don't understand why the older 28-70mm f/2.8L is in the A group (the best AF sensor performance) but the newer 24-70mm f/2.8L is in the B group. Of course all the f/1.2, f/1.4, f/1.8, and f/2.0 lenses are all in the A group. The 70-200mm f/2.8L are all in this group as well along with 28-70mm f/2.8L. There's no mention of then24-70mm f/2.8L II.

So why are the 14mm, 15mm, 20mm, 24mm and 24-70mm that are all f/2.8 all in Group B which only activates the the one center Dual Cross-type Point instead of 5 points in Group A?

Why do you think this is?

I bet the 24-70 mkII will belong to group A. I guess that's how Canon justifies the high price and convinces everyone to upgrade.

Why kno help here? Is there some place at canon to ask this question? Maybe I should try another forum

Light falloff perhaps?

I don't have an answer for you, but the common theme appears (from your list) to be 24mm or wider. I'm no lens designer, but is is possible that wider angle is harder to get good focus contrast or something like that. I know the DOF will be greater at 2.8 for wide lenses.

Suprised I haven't seen this come up yet on any other forums. The great AF update on the MK III is onyl going to work with the new $2300 Canon 24-70 2.8? So those of us with the old $1300 24-70 2.8 USM I are gettig one central AF point like the 5d MK II ? Hopefully Im just confused and this isnt the case.

Canon might be convincing me to upgrad....Upgrade to a Nikon 24-70 and D800. I might have an excellent Canon 24-70 2.8 for sale shortly!

jpkrze wrote in post #14174793
Suprised I haven't seen this come up yet on any other forums. The great AF update on the MK III is onyl going to work with the new $2300 Canon 24-70 2.8? So those of us with the old $1300 24-70 2.8 USM I are gettig one central AF point like the 5d MK II ? Hopefully Im just confused and this isnt the case.

Canon might be convincing me to upgrad....Upgrade to a Nikon 24-70 and D800. I might have an excellent Canon 24-70 2.8 for sale shortly!

The only thing you lose is 4 of the 5 dual cross type sensors in the middle, the center dual cross type + the other 40 regular cross type sensors work fine...

dbdors wrote in post #14174595
I don't have an answer for you, but the common theme appears (from your list) to be 24mm or wider. I'm no lens designer, but is is possible that wider angle is harder to get good focus contrast or something like that. I know the DOF will be greater at 2.8 for wide lenses.

Db went back and looked at the manual again and you might have hit the nail on the head. The only lenses wider than or equal to 24mm that works is the 24mm f/1.4L

I don't know the Nikon products well enough to know if they have similiar limitations. Also this only impacts the dual cross-points. I was looking at the 1D Mk IV and I don't believe it has dual croos type sensors so it doesn't suffer it's problem. I will have to investigate more.

Actually found info that explains this more.... It was found on the CanonRumors site. FM and Other sites appear to be more up to speed on this.

"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."