GIYF - found part of an answer in another thread:"Be aware that the camera uses one AF system for when you use the viewfinder, and another AF system when you use Live View mode."https://community.usa.canon.com/t5/EF-RF-Lenses/Canon-EF-100-400-f-4-5-6...
I have the same problem with that lens when shooting through the viewfinder, but what I found odd is that on my 1D-X MkII when I shoot using liveview / the backscreen, AF does work! Granted its slower to get there than the 1.4x III, but it is possibl...
