2X extender on 5D, Mark ll

Don't know why, but I tried 2x canon extender with 180mm macro 3.5f and 100-400mm and the autofocus (back button) works when on the 5D, Mark ll. Tried with 40D and, as expected, did not work. Thought that this might interest you.

I believe the Mark II 5D can AF at f/8 which would explain this

I tried the 2x with my 100-400 and it did not work?? Best I could get was f9 with zoom all the way in.
Why would it work with yours???
Just curious.

Here is a better description of the situation; autofocus with the extender only works with Live View. AF mode setting is "Live mode". thus, the mirror stays up. When AF mode is set to "Quick Mode", autofocus does not work. Live View function setting includes "Stills Only". When using on 180mm macro it does a reasonable job of searching for the focus, albeit a bit slow and sometimes misses the mark. On the 100-400mm search and focus seems to work at various mm of the lens.

Wyn,
Thanks for the further info. Yes, indeed, it does work. Wonder why canon never mentions this? Of course, it is kind of slow, but still useful. How cool!
Greg