difference in how well they will AF with lenses of the same f-stop? I have 3 lenses that I use my TC's on: 1. Nikon 70-200 f2.8 AFS VR 2. Sigma 120-300 f2.8 HSM 3. Sigma 500 f4.5 HSM I have the following TC's: 1. Nikon TC-14E 2. Nikon TC-20E 3. Sigma 1.4 All three lenses work really well, no really noticeable focus difference, with the Nikon TC-14E attached. The Nikon 70-200 works great with the TC-20E as well. The Sigma 120-300 almost works with the Nikon TC-20E, it "gets close" after some hunting, it works great with the Sigma 1.4. The Sigma 500 does not work with either the Sigma 1.4 or the Nikon TC-20E. It has always been my understanding that the limiting factor is the "cumulative" f-stop after adding the TC. So, for example, on the 70-200 the TC-14E yields an f4 (+1 stop) and the TC-20 an f5.6 (+2 stop). Given this, what other factors might be inolved that cause the TC-20E to not work on the 120-300 f2.8? On the 500, adding the TC-14E yields f4.5 + 1 stop, f6.3 or so I'd guess, so I understand why the TC-20E would not work as that would be f9. But why would the Sigma 1.4 TC not work? I will, in the near future, be picking up the TC-17E which I expect to work fine on all 3 lenses, but I am wondering about what other factors are in play here that I am not aware of. Thanks in advance for the education.