Originally posted by vonBaloney 1. Then shouldn't the same thing happen with all lenses with a TC that passes through the focal length?
2. It should mess up on all lenses fast or slow because the lack of light is not the problem, the incorrect calculation is.
3. (Why have AF capable TCs at all if they can't function by definition?)
Anyway, that has not been my experience -- fast lenses on a TC autofocus almost as well as without the TC.
4. Autofocus gets progressively worse the less light there is -- we know this to be true even if you are right so it is certainly *a* cause if not *the* cause. Expecting autofocus to work reliably at an effective f8 -- it just isn't going to happen. (Of course it is not just the stop value itself, but the actual light of the scene is very important also -- f8 may be OK on a bright sunny day.)
5. But what evidence is there that the camera "knows" the scale of response? The focus throw is different for every lens, even of the same focal length. Does the lens transmit this information also? I've never heard mention of such a thing.
1. Yes.
2. It should.
3. There are AF capable TC's. I've used them with long lenses. I'm not sure if the K-5 does some funny things with the 300mm to improve the AF that it doesn't do with, say, the sigma 500mm f/4.5 + TC. Perhaps there's some algorithm embedded in the camera.
I'm not saying that this scenario makes sense to me. But it is without question in my experience that the AF of the 300mm works differently with TC than any other lens I've tried TC with. (Really only the sigma 500mm f/4.5 and the sigma 300mm f/2.8). At least two other posters have complained about an 'odd' overshoot as well with the Tamron + 300mm.
4. Of course.
5. Again, I don't have a good explanation so I'm defaulting to this 'maybe the camera is doing something weird'.
In summary - I don't expect this to be a satisfying answer to you. I don't know if I'd believe you or find you credible if our situations were reversed. If you have access to a 300mm, I'd suggest you try it - perhaps it's just a problem with my setup... but I doubt it.