From my perspective, I never really ever shoot in a situation (2x extenders) and I try to get as close as possible to my subject matter that it hardly becomes an issue. Not to say that I wont ever get in that situation and using a 2x extender on a F4 lens wouldn't be nice to use and I know if i had to use that combo I'd want AF and would be frustrated if I couldn't, BUT, it is what it is. I guess, for me, from the early days at college when i could use/borrow any lens/extender/camera I needed for any assignment, it never sat well with me I'd be losing more light and I thought it was stupid so i made the conscious decision to not use/need extenders. Oh well, the 1dx and 5d3 are still awesome cameras to me so I'll be grabbing mine asap.