I bought a Kenko pro 1.4 TC to use on my 100-400mm on my 7D because its supposed to auto focus. It does, just not very well. I was out on a clear sunny day and the shots that I got it to work on were great, very sharp. I take bird shots and a majority of the time it would hunt badly and not focus. At one time I was trying to take photos of waterfowl on a pond and it would not focus no matter what. I'm was very happy with the results when it worked but I can't shoot birds fighting it like that. I was using single shot and spot focus, should I have used some other settings or should I make someone a good deal on a slightly used TC.