I've used the 2x mkIII on a 70-200 mk II and I feel like I lost speed and precision on the auto focus shooting outdoor sports (soccer). Keeper ratio went down significantly. Could have been a byproduct of the longer reach though. I have the MkI version of the 100-400. In good light I shoot it handheld, and it delivers. I've heard the mk II is even better (and isn't a "dust bazooka").
Last edited by ChrisF; 03/07/17.
