20D & Sigma 70-300 APO

[connolo]

Anyone else with this combination?
I am having issues with focussing. I've had this lens for a few months but I've only noticed this recently as I tend to use set the focus point to the centre point for most of the time. However, when the focus selection point is set to automatic this lens struggles to focus and intermittently but frequently the focussing motor gets stuck and whirrs away for as long as the shutter is half depressed. Looking at the lens it stops at the closest focussing point and it sounds like the motor is trying to force the lens to focus closer still (that whirring doesn't sound good). This only happens when focus point selection is automatic (works fine manually setting any of the other focus points), and it happens in One Shot, AI Focus and AI Servo.

I don't have this issue with any other lenses, though this is the only telephoto I have. I don't even know if the issue is with the camera or lens. Anyone heard of this annoying little feature?

Cheers,
Ollie

[pharris]

My 70 - 300 APO used to do that on my EOS film cameras, I never got to try it on my 20D as it gave the old error99 on my 10D. Its probably a fault with the lens or rather sigma not reverse engineering the mount properly.

Paul

[leesc_1998]

I think it's a case of "confused lens" that's causing the problem you're describing. If you are trying to focus on something in the distance with a lot of different colors the lens is going to try to find the "best" spot to focus on. It wouldn't take much movement to trigger the lens trying to focus again.

I used that lens to shoot a soccer game a couple of weeks ago and it performed flawlessly. I was shooting on the pre-set action setting and auto-focus. It would refocus as I panned shooting on burst mode keeping up even when the players were coming straight at me.

Now, having said all of that, there is always the possibility that there's a problem with your lens. The only way to tell for sure is to take/send it in for service.

[connolo]

Thanks guys.

leesc, were you using 20D with the lens? If so, was it set on automatic focus point selection? On manual focus point selection this lens performs really well as you say it does.

[Wazza]

I just posted these shots yesterday which were with that combination:

http://photography-on-the.net/forum/showthread.php?t=72509

I generally shoot centre point focus, unless I'm shooting group object shots, such as people, where I'll use all 9 AF points. So it will hopefully pick an average focus point..

Generally the lens will search for a focus, when it's either too low light, or you're on a very smooth surface with no edge. It's always a lot easier to focus on the edge of a sharp edge or logo, than something round and shiny, such as a car window. Instead, try locking onto the door handle, and then reframe the shot.

[connolo]

Wazza, I think I must have a faulty lens. In the difficult focus situations, the focus ring locks on its shortest focus and the motor sounds like its trying to force the lens to focus even closer. When its hunting (and can't achieve focus) it should (I believe) move the focus ring all the way to the shortest focus, then all the way back to infinity, then stop and the camera signals that focus hasn't been achieved.
Can you confirm that your lens doesn't behave the same as mine?

Just to confirm the lens is the Sigma 70-300mm f4/5.6 APO Macro Super 2