I have a Sigma 24-250mm lens for a Canon. The AutoFocus doesn't work. When it is switched on and I half press the shutter release, the lens goes to near, to infinits, to near and stops. I have tried it on my old EOS500 (35mm) and my new EOS550D and it is the same on both.

Is there a common way to repair such a problem, or is it going to be a professional repair job? The lens is about 10 years old.

First up, try cleaning the electronic contacts on the lens where it mounts on the camera.

Second, try the various focus point settings on the camera to see if it makes any difference. I ended up using just the centre focus point and reframing because the lens was trying to be cleverer than me.

That isn't difficult in my case.

Don't be so hard on yourself.

You're just as brainy as a camera lens.

Same thing happened to my Sigma 50mm macro lens - my local place tried twice to fix it (I thing the focussing gears in the lens were stripped) without success. In the end I dumped it and bought a Tamron 90mm macro instead.

All I can suggest is send it back to Sigma - if anyone can fix it, they can.

Hmm, thank Dogbert. Given the age of the lens, I might use it with manual focus.

I took a lot of photos at the weekend, where the autofocus couldn't focus properly on my other lens (shooting birds in cages at the botanic gardens confused the hell out of the camera/lens).

That sounds pretty challenging anyway - I'd expect it kept shifting from the bars to the bird to the background?

Yep, pretty much. Quickly flipped onto manual focus and concentrated on what I wanted. A couple of samples are in the Random Photo thread.