40mm pancake only has full time AF on T4i

Which is a known fact, but I have people argue with me saying that it will also AF during video (continuously) On 5DM3. I know it is not true, but can anyone point to an official source that I point to to silence these folks???? My word is not enough. Canon itself would be there best. Thanks

The 3 bodies that could do the firmware update on the 40stm were the t4i, 5d3 and 1Dx. So it wouldn't be impossible for the 5d3 to autofocus in video would it?

They are talking bollox.
The 5DMKIII (or any other Canon body) will not AF continuously during video. It doesn't matter what lens you put on a 5DMKIII its never going to happen.
Point them to the press release of the 650D where it mentions it being the first of its kind.

Yetihenry wrote in post #15277111
The 3 bodies that could do the firmware update on the 40stm were the t4i, 5d3 and 1Dx. So it wouldn't be impossible for the 5d3 to autofocus in video would it?

Yes it would be impossible. Do you know why the 650D can do it?
Its because it has a brand new type of Hybrid sensor. Its nothing to do with firmware or any sort of hack. Its plain old hardware.

Yep of course. Hybrid sensor it is obvious. However I wish canon 40мм discription stated something to the extent "AF in video only supported with t4i" or similar.

Yeah, I find it a bit confusing how Canon markets this lens.
It's been designed for the 650D yet it's not a EF-S lens. It's designed, or optimised for video, yet has no IS which would be great for hand held video.

Still, it's a fantastic lens. I love mine.

it seems like the 40mm MF ring only works with camera in on position like the 85mm 1.2

I believe it is the DSLRs mirror that is on the way of ability to AF for video.
Exceptions are Sony's cleaver Transparent mirror DSLRs that have no impact here.

fiveFPS wrote in post #15277352
it seems like the 40mm MF ring only works with camera in on position like the 85mm 1.2

Yup, I believe it's fly-by-wire MF. (ie: there's no physical link like gears between the MF ring and the motor...you simply turn the ring and it sends the AF motor a message saying pull forward or backward).

I've also found that if you use the rear AF-on button for focus you get full time manual focus without having to switch the lens to M or press any buttons. Quite handy.