1Dx and f/8 AF: it works.

[Childeric]

I have just tried the following combination on my production 1Dx :

Canon EF 70-200 f/2,8 II IS + Canon extender x2 III + Kenko extender x1,4 pro 300.

With these attached, AF works well, just a bit slow, with the central sensor. But exposure is wrong, roughly 2 IL overexposed.

It also works with the Canon extender x2 II.

[Mt Spokane Photography]

Without taping the pins??

[neuroanatomist]

Without taping the pins??

With the non-reporting Kenko behind the Canon 2x, that makes sense. The same Kenko 1.4x allows AF on f/4.x-5.6 zooms.

[Childeric]

Yes, without taping the pins.

[Childeric]

AF also works on the same 1Dx with the same Kenko x1,4 attached to the 70-300 f4,5-5,6 DO, at all focal lenghts.

Same issue with exposure however.

[Childeric]

From what I collect from various forums, my version of the Kenko x1,4 is reporting. And by the way, aperture as shown on the 1Dx display is the right one (minimum is f/8). Therefore, I am puzzled why exposure is wrong by more than 2 IL. Anybody would have an explanation ?

[Childeric]

After more testing, it seems that it is overexposed by 5/3 IL (1,67).

Even odder...

[Mt Spokane Photography]

I tested my 5D MK III with 2 TC's, and it worked with a 70-200mm f/2.8 but slowly.  Other lenses ranged from not at all to unreliable AF.  I also found AF to be inaccurate.  The in focus light flashed and the shutter opened, but images had varying degrees of AF accuracy.
I decided it could not be trusted except on the 70-200mm f/2.8, and with 2 TC's stacked on it, IQ was far from what I'd want with a huge investment.
Also note that when you stack TC's, only the one closest to the camera reports, with Canon TC's, it reports the wrong aperture and focal lenght.