A 2x teleconverter would bring the max aperture to 8, and if the teleconverter has the chip to inform the camera of such, you will lose AF like you said. (on the 1 series and EOS 3 cameras AF is possible with a max aperture of 8, but only on the center AF point iirc).

Some cheap 3rd party TCs don't report the change in effective aperture so if it's bright enough you can hopefully continue to AF without a care in the world. But a lot of those TCs have significant distortions edge to edge as well as reduced sharpness. If you can't afford the Canon (and I can't blame you) Kenko I know makes some decent ones that cost considerably less. I use one myself, instead of buying a Canon 2x I bought a used near top-of-the-line Kenko 2x (new price is around $230 maybe) for about $150. Optically, it's maybe a little softer, especially at the edges, but the performance is quite close to the Canon IMO and the price is hard to argue with. I don't use it enough for anything more to make much sense.

Another benefit of the 3rd party ones is you can use them on any lens. If I bought the Canon TC it wouldn't fit on my macro lens.

A friend of mine has a really cheap 2x TC and it didn't report aperture correction (so my camera was AFing at F11), it was also very soft and introduced a lot of chromatic aberrations.

One of the UK photo mags just did a review. They actually said the Sigma EX DGs were better than the canons, and the best in the bunch. They also tested the kenkos and something else and they weren't as good... I personally own the sigma EX DGs (both the 1.4 and the 2.0). I don't know any different, but using the 1.4 with the sigma 70~200mm f2.8 works VERY well. Slight IQ hit wide open, but by f/4 it's very good. The 2.0 has noticeable IQ loss until you're 2 stops down (so f/8) so I only use it when I must. They were bought when I went from Bigma to the 70~200mm in case. I tend to shoot with that lens with the 1.4 at least 50% of the time, just the lens 40% of the time, and the 2.0 maybe 10% of the time. I've wondered if I should have gotten the 100~300 f/4 but then I couldn't have f/2.8 at 70~200 when needed.

I just moved all my galleries off pbase, but I'll try to link to them when I can.

Ben

FWIW, I just ordered the Sigma 2x from sigma4less.com this morning to go with my Sigma 70-200 2.8 for $195 shipped. AFAIK, the Sigma TCs are only designed to work with Sigma lenses. Someone else might be able to comment/correct me on this.

I'll probably get the 1.4x later, but I wanted the extra reach for an upcoming trip to the San Diego Wild Animal Park. Also, losing 2 stops wasn't as big a deal from f/2.8 as it would be from f/4.

I've read good things about the Kenko/Tamron PRO series. I looked into them until I realized I could get the Sigma cheaper, and it was compatible with my telephoto.

If I wanted to get a good one, I'd be getting the Sigmas, personally.

the sigmas are compatible with MORE lenses than the canon's... Basically the canons just work with canons. Sigmas work with at least canons and sigmas, not sure about other 3rd party lenses though.

PotN has a great FAQ on them here:
http://photography-on-the.net/forum/...ad.php?t=41922

The Canons only work with certain lenses, too.