I recently picked up a Tamron 2X non reporting TC. My intent is to use it with my 70-200 4L. After playing around with it I found some results that maybe some of you could explain.
First, I am using this on a 5DII.
1) 70-200 4L: Surprisingly AF works great. There are a couple misses, but I am pleased with how quick and accurate the lens focuses. Even in evening light it seems pretty quick.
2) Sigma 50 f/1.4: Now, I don't think I would use the 2X TC with this lens, but I tried it anyways. I thought that with the aperture being f/2.8 AF should have no problem. Quite the opposite. Without the TC, AF is quick and most of the time accurate.
With the TC AF hunts and jitters back and forth and only sometimes does it obtain proper focus. I've tried it in very bright light and that doesn't help. If I put the lens to manual focus, the focus confirm light works properly, but AF just doesn't want to work.
Any ideas why the 50mm f/1.4 is having trouble with the TC? I mean even at f/2.8 the high precision centre point should still be activated. I tried using the outer focus points and while still pretty bad I think they work a little better (this could be my imagination though).
Anyways, I thought some of you may know the answer or be interested in my observations.
I will try with my 17-40 4L tonight and see how AF works any better than the sigma.