Quote Originally Posted by conropl View Post
There was a firmware change on your 1DX that allows you now to AF with a f/8 lens. Is f/8 the limit, or can you go beyond that? Without going beyond the f/8 AF capability, the 2XIII extender can not be used on the 800mm.
The limit with a Canon TC is f/8. I'm not sure how a Kenko 2x would do with an f/5.6 lens, but given the optical hit with the Kenko 2x (much greater IQ detriment than the Canon 2x), I'm not sure it's a good option anyway. Note that's for phase detect AF - contract detect AF works at f/11.

Quote Originally Posted by conropl View Post
If the next generation 800mm could get to f/4, then that would be a game changer by allowing the 2X to get even more reach (even at a degraded IQ, some would bit to get the longer reach).
It would also be a game changer by being approximately the same size as the EF 1200mm f/5.6. I doubt we'll see such a lens...