Sigma 135-400 +teleconverter?

Has anybody had any experience with the sigma 135-400mm lens and a teleconverter

of any kind? Does anybody know of any teleconverters that will be compatabale

with this lens hopefully retaing AF?

Its not the teleconverter that retains AF, but rather the body. With anything other than an EOS 1/3 series body, you'll lose AF with any teleconverter. However, if you tape some of the contacts (the two large ones, I believe), the camera will TRY to autofocus, but it won't work very well.

woops, left a bit out: Taping of contacts is only necessary with a Canon TC. With a third party teleconverter (I.e. Tamron or Sigma), it will always try to AF(Emphasis on try), since it doesn't actually communicate with the body like Canon's do.

I think any AF Canon compatible TC will need to have the pins taped to retain AF with that lens. I have to tape the pins on my Kenko Pro 300 for the AF to work with a Canon 100-400.