The limitation is that most Canon cameras can only *autofocus* with maximum apertures up to f/5.6. Because the 100-400 + 1.4x combo has a maximum aperture of f/8 (at 400mm), you generally can't use autofocus with that combination. The exceptions to the rule are the 1-series cameras, the 5D mark III and the 7D mark II.
This morning, I found the battery dead and The autofocus on my camera doesn't work (I don't know if the lens autofocus is bad, because I only have one camera with me, but i'm sure the camera autofocus is bad, because I have several lenses. Auto focus on my Canon 70D is not working in P,T, A, and M mode, while working perfectly in Auto, CA and SCN mode. I have to press the back button AF-on to focus when taking a picture. But if I use live view the auto focus is working normally. I believe that pressing the shutter button half way should always start focusing, also for the T, A The 18-55 STM, and 18-135 STM (or 18-135 USM) have virtually silent AF that does not move during AF or MF. The 50mm lens has a noisy AF that moves during AF or MF. With both the 18-55 and 18-135, any AF or MF movement takes place inside the lens and is not visible or audible to the user. The Canon EOS 5D Mark IV has a huge userbase, with popularity in commercial, portrait, and wedding photography. Among other accolades, the 5D Mark IV is considered the best Canon camera for people photography thanks to its impressive dual pixel autofocus system, with wide 61-point coverage and solid 7fps continuous shooting speed. 2lk566.