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I have a Sigma 105mm macro lens (Canon fit), had it for 3-4 years (can't quite remember when I got it) but use it a lot and noticed it either won't focus on auto focus or it starts to focus and then sticks. Manual focus is still fine. Anyone know what the cause is?

Do you mean it focus hunts? Or doesn't even try to focus? If the former, is it only in low light or low contrast?

Have you tried cleaning the lens contacts?

When I use it, it will focus fully out at times (in any light situation), then it slows way down as it focuses back in and focuses way down and sticks at infinity and won't focus again in AF.

Hi Dave....Could be the focus lock Dave. I am always locking my ruddy lense up. The focus ring pulls back and forth to lock and unlock the lenses range. It is handy if you dont want the lense constantly going through the full range to find focus....however...I have never used this seemingly useless feature as it seems more of a hindrance than a help.

Iain.

Thanks Iain...you mean the switch that says Limit or full? If so, it isn't that, I always make sure I set it to full, often have to double check I switched the MF/AF switch and not that one, which I also see no point of having a limit switch on the lens. Isn't the focus ring either.

Hi Dave, are the pictures coming out fine, your not getting any all grey images or anything like that.

Regards Gruditch

Sadly, it sounds as though the rubber belt, which drives the AF, has become slack. You can read all about it at http://www.camerahacker.com/sigma_105_ex/Servicing.php. The key point is right at the end. It should not be expensive to have the belt replaced, if you can find a co-operative repairer. Do NOT use 3 in 1 oil, though!!! I only use silicone oils on plastic gears.

Apart from that, I think the only hope is checking the electrical contacts on both lens and camera, as dilletante has already suggested, since dirty contacts could reduce the power available to focus the lens. I use one of those type-eraser pencils for contact cleaning.

I assume all your other lenses are working ok, which eliminates a fault in the camera.

Mike

Gruditch wrote:Hi Dave, are the pictures coming out fine, your not getting any all grey images or anything like that.

Regards Gruditch

The images appear fine, its now not moving well on manual focus either.

Sadly, it sounds as though the rubber belt, which drives the AF has become slack. You can read all about it at http://www.camerahacker.com/sigma_105_ex/Servicing.php. It should not be expensive to have the belt replaced, if you can find a co-operative repairer.

You might be right about that. I'll have a look inside it as the link shows and see.

Just opened up the lens from what it says in the link, it looked a little different inside than the one in the image, the motor isn't the same, has no belt on it or gears.

Dave McCormick wrote:no belt on it or gears.

Oh dear! It just goes to show "never believe what you read on the internet!". Is there anything that look as though it could do with a bit of lubrication?

Mike

This may sound stupid - but why don't you just send it to a Sigma repair centre, who will quote you before making any repair?

Cheers,

- Pete

Oh dear! It just goes to show "never believe what you read on the internet!". Is there anything that look as though it could do with a bit of lubrication?

Nope.

Pete Eeles wrote:This may sound stupid - but why don't you just send it to a Sigma repair centre, who will quote you before making any repair?

Cheers,

- Pete

Think I might do that. Just didn't want to be without a macro lens as I don't have any other lenses or cameras that can take macro shots at the moment but I suppose a little wait for a working lens would be worth it.

I've just picked up this thread and maybe a bit late but for future reference, I can only recommend thay you send your lens to the UK Sigma repair centre. I sent my telephoto lens for repair in April this year and they gave me a superb service: 1 week in all. They do not overcharge either, you can ring them anytime to check on the progress, and they are very reliable. NEVER try to repair the lens yourself as you may end up making more damage...
You can find all their details on their website: http://www.sigma-imaging-uk.com/support/index.htm
Send your lens with a covering letter explaining the fault and do not forget to mention which camera fit you use the lens on as they will need to test it to check what is really wrong with it. You can ring them before sending (ask to speak to Daniel who is the chief technician and is a very nice chap) !!
Sylvie

Been a while and have not had much time to do much about it but decided that would be the best option so going to do that. Thanks Sylvie (and everyone else) for the advice.