Kenko TC and AF at F8

[Mt Spokane Photography]

I think that the system was not intended to do well at f/8, but they decided that it was good enough for most usage.  Certainly, my 5D MK III did very well with the Kenko 1.4X and 100-400 while at 400mm.  At least until the Kenko died.  Everything is working fine since I removed the TC.

[neuroanatomist]

There was some focus hunting when shooting birds in dense foliage with backlighting

Here's a shot that shows the bird and surround, a tough job for any AF system.  This was with phase AF, and I haven't done an AFMA yet (need to find a deserted football field for that!).  Might be a little back-focused, I think.

Shot is with the 1D X and 600mm f/4L IS II + 2xIII, 1/320 s, f/8, ISO 4000.  Original and a 100% crop to show detail, which is quite good, IMO. 

[Mt Spokane Photography]

Wow, the backlighting is tough to deal with.  It might be a hair backfocused. 
I had the older non is 600mm f/4.  It was too much for me to haul around and setup, but it did a great job.  I finally resigned myself to making do with my 100-400mmL, and its pretty good.
 

[WesternGuy]

Neuro, wow - that looks great - I can hardly wait to get my 600mm II.

WesternGuy