Canon cameras, lens and Teleconverters

Recently I purchased a Canon 70-200mm F2.8 L lens to use for sports photography. Before purchase of the lens I owned 2 teleplus extenders, C-AF 1.4X MC and C-AF1 2X Teleplus MC4. The 1.4X works fine but the 2X is unable to auto focus. This poor performance of the 2X surprised me as I thought with a loss of 2 F Stops would bring this lens to F5.6. From what I have been told this F5.6 is within Canons range for TC to work with autofocus. I have 2 Canon cameras 40D and 550D both perform the same. Has anyone have any suggestions as to why the 2X does not work in autofocus?

I don't galelegg, as you surmise, it should at least try to AF with the 2X @ f/5.6.

I wouldn't expect great results, but it really should work. All I can imagine is that Canon does not seem to like that particular 2X converter. That said, usually Canon won't notice a 3rd party converter like that and will at least try and AF. So it's not normal behavior you are experiencing.

I tried the 2x with the 70-200 2.8 II on my 6D......the AF did work, but it was slow to the point of being useless. You could stand there and wait for it to come to focus. Maybe 2-3 seconds.

So, for all intents, it doesn't work. At least for me.

Just get the Canon 100-400 II. That one works just swell.

I sold my 2X as it didn't work well at all with my 70-200 f/2.8.

Back when I had my 70-200 F2.8 L IS (Mk1) and Canon 2 x Mk2 they played quite nicely together. True there is a loss of IQ (but not silly bad) and the AF slows - but 2-3 seconds?!? Even with this setup on my 5Dc I could get a decent proportion of shrap-ish shots on Red Kites in flight and an adequately high keeper rate on Motorsports. Naturally on my (then) 1D4 things got rather better.

Certainly not an ideal setup but quite workable.

I have the EF 70-200 f2.8 MkII paired to a Yongnuo 2x. The AF response is quite fast. I just have to compensate exposure by 2 stops (some sort of a bug in the Yongnuo, but it is a constant).

My wife shot this combo with the 60D in an African photo safari, with excellent results!

Thanks to all that replied. Seems that TC and Canon lenses at least are not that straight forward. Before I bought the Canon 70-200mm L 2.8 lens I was considering the Sigma equivalent I would be interested to know if anyone has tried the Sigma lens with the same TC's. To date I have found the 1.4 TC works well with my Sigma 150-500mm F6.3 and my Tamron 18-270mm F6.3 both giving a reading of F9. The TC x2 only works in manual focus but the Sigma and Tamron still gives a good result. To me Canon lenses are overrated both Sigma and Tamron are much the same and about 40% cheaper. Perhaps Canon would be preferred if your job depended on it but for amateurs I seriously doubt especially if your photos are viewed on a TV monitor or worse still on a mobile phone.

Thanks to all that posted replies. I just purchased a Canon 70d and have found both the 1.4 and the 2x converters work with autofocus in Live view mode on both Sigma 150-500mm f6.3 and Canon 70-200mm f2.8 l lenses. Best settings on camera was spot focus and spot exposure. Focus was reasonably fast great combination. I assume the 80d will work just the same. I would be interested to know how Canon 60d performs.