Quote Originally Posted by jrw View Post
EDIT: Checking back for info on the v3 extenders I find 50 and 75% ratings for the impact on AF speed. I suppose this is a testament to the speed of the AF drive in the 300 mm lens that I never noticed any practical difference.
I haven't noticed the AF speed reduction either and have used the 2X III + 300 2.8 II for photographing actively feeding hummers w success. I guess it might show up in my fast dog application in the other recent thread on 5DMK III AF performance. However, I'm not likely to run the experiment - too many variables, too little time.