Someone please explain this f/8 brouhaha to me: my 5D3 + Kenko 1.4x tele-extender + 400 & 70-300 auto-focus (at f/8, pre-firmware update) all day long. I just downloaded the firmware update and will install it later today, but I've been auto-focusing just fine at f/8 for some time. Am I missing something?
You are missing ... Canon teleconverters which report the correct maximum aperture to camera.
When you put kenco with your say 400 f/5.6 lens does it report it as having a maximum f/8 aperture? I guess not! It still reports it as f/5.6!
You have effectively a 1.4X400 = 560mm f/8 lens but the camera thinks it is connected to a f/5.6 maximum aperture lens and tries to autofocus!