Thanks to everyone that replied to my previous question. I now know just enough to be dangerous..!

When using the Elan 7E and a 50-500 Sigma that has F stops of f4.0 to f6.3, AND if I have the lens zoomed all the way out to 500mm, I think that I only will have an aperature available that is 6.3 which is a smaller opening than the 5.6 supposedly required for AF.

Is it likely that the AF will not work at max zoom, or is the f4.0 still somehow available for AF purposes only? Perhaps there is a good likelyhood that the AF will hunt if the available light is low...??

It would appear from Sigma adds that there isn't a problem on AF with EOS and this lens but of course I wouldn't expect them to point it out..! Sigma does say that the lens does not AF with the 1.4 or 2.0 TCs, but will manually focus ok.

Sorry to be such a bother but it is expensive to learn this stuff by buying things in error to do it..!

-- Venado Loco (deertex@swbell.net), January 26, 2001

Answers

This lens isn't the only one that is slower than f:5.6 when zoomed to it's max. The Sigma & Tamron 28-300's as well as Vivitar's new 28- 200 all go past f:5.6 at their long end. They all still AF on EOS cameras though. They just lie to the camera. They all report to the body that they are f:5.6. The camera doesn't know any better so they try to AF. As long as light & contrast levels are adequate it works.

The same thing happens when you use f:5.6 lenses with a 1.4X TC. Canon has set the AF cut off at f:5.6 for good reason. The AF performance suffers greatly at smaller apertures. But since an effective aperture of f:8 sort of works (f:5.6 with 1.4X TC) the f:6.3 zoom will be even better, but still not very good.

By the way, f:5.6 lenses usually hunt sometimes as well, so f:6.3 will most certainly do it, unless the lens focusing motor is very slow.

-- Jim Strutz (jimstrutz@juno.com), January 27, 2001.

Some of the Sigma telephoto zoom lenses for the Canon are actually f/5.6 instead of f/6.3. I saw this in a lens test of the Sigma 170- 500mm zoom.

I think it has to do with the larger diameter lens mount opening.

-- Marc Bergman (mbergma2@ix.netcom.com), January 27, 2001.

Venado L: I do have the sigma 50-500 EX HSM, and I'm using it in an Elan IIe. I'm not able to explain the technical information you want ( I'm a beginner, too) but I can say that the autofocus works great...very fast and quiet. I'm really happy with this lens. It's very well built and sharp. Ohh, and I have read a comment ( I can't remember where) about a guy who owns this lens and he is using it with a sigma 1,4X, and the autofocus still works ( I don't know....maybe in a completely sunny day ?????). Good luck.

-- Nicolas (nicodelo@infovia.com.ar), January 28, 2001.

This is a related question. Canon TC's report the aperture to the camera, disabling AF if greater than F:5.6 (with most EOS cameras). I've heard some non-Canon TC's either don't report aperture or lie to the camera. I'm interested in a 1.4X TC for my 100-400 L IS, but would like to retain AF, even it's slower and less reliable. Anyone have any experience with non-Canon TC's for this application and could perhaps advise what TC to buy? Thanks, MIKE

-- Mike Raub (mraub@hrva.com), January 28, 2001.

Mike Raub, I use a Kenko TC with Canon's 75-300 and 100-300. It works if there is enough light and enough contrast, but it hunts a lot. This will be the same with any non-Canon brand 1.4X TC. It's not likely to work with any 2X TC though. Also the image quality will not be degraded as much with the 1.4X as it would be with the 2X.

-- Jim Strutz (jimstrutz@juno.com), January 29, 2001.