If you try to use the Sigma 1.4 AF converter on a 400 f5.6 AF lens, (Sigma or Tokina), it will not autofocus unless you tape over one of the electrical contacts on the converter. You can retain autofocus with either lens using a Kenco or Tamron 1.4 AF converter. Build quality and optics of the Tokina 400 f5.6 ATX SD lens is far superior to the Sigma of the same spec, I've had both! I now have a Tokina 80-100mm f4.5-5.6 ATX Zoom, a great lens, in a convenient compact size, very sharp and contrasty, close focussing and complete with a close-up filter lens which reduces minimum focus to approx 1.5 metres. Well worth considering.

nirofo.

Not sure I would use a TC with this lens. Most side by side reviews I have seen put the Sigma APO Macro above the Tokina. There are many variants of the Sigma though, and the APO Macro is the last, and best. The build is pretty decent, certainly better than I expected and akin to a mid-range Nikon, maybe better due to extensive metal. A problem with these independents is said to be significant sample variation.

I have yet to see an image from one of these independent 400m lenses which has the sort of bite you see in images from the best primes, such as the 200mm F4 AF micro (which I own), or the 200-400mm F4 AFS VR.