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?
The Kenko locks up the 5DIII when used with the latest generation telephotos, and you have to remove the battery to restart.
I found this to be true ONLY when AFMA is enabled. Otherwise, Kenko TC's autofocus fine at f/8 and did so prior to the 5D3 firmware update.
My car runs fine, except when I try to back up while turning - then the steering wheel locks up. But otherwise, it works fine. Maybe that fits some people's definition of 'works fine' - not mine.
But hey, if locking up with AFMA means the Kenko TC 'works' then you could also say the Canon 1.4x 'works' with the 70-300L. If you zoom the lens out to the 250-300mm range, the 1.4x TC can be mounted. It will AF on the 5DIII, you can AFMA the combo, and your camera won't lock up. Retracting the zoom will bump the rear lens element into the rubber ring on the TC, long as you keep the zoom extended, it 'works' just fine.