A friend of mine has a Sigma 300mm f/2.8 HSM prime lens (Canon mount) that's several years old. It used to work perfectly, but lately, the autofocus mechanism occasionally just won't work--as if the lens was switched to manual focus mode. Sometimes the lens will work fine, but the next time you mount it on the camera, it won't even try to focus. No noise, no errors, no nothing. He sees this behavior on his 1D-series bodies, and I've seen it on my 50D body.
He'd like to sell the lens, but neither of us has any idea how serious this problem is. I don't believe he's contacted Sigma about the problem yet. Google didn't turn up anybody else complaining of this issue.
Have any of you heard of broken auto focus on a Sigma lens, especially on the 300/2.8? Any ideas what might be causing the problem? We've already cleaned the electrical contacts, but to no avail. My own standard response to this sort of problem is to dismantle the lens to look for obvious problems, but I haven't tried that yet.
I've got the same lens, but mine hasn't had any problems. Sometimes I bump the switch to MF and it takes a second to realize it, but otherwise no issues that aren't user error. If that's not the case, I'd look carefully at the switch. Probably not completely engaging in one setting or the other.
I wouldn't sell a lens with an intermittent problem...just not how I'd operate. Just passing on a problem to the next guy. If looking at the contacts on the switch didn't sort it out, I'd send it in and let Sigma check it out.
The owner certainly would disclose the problem before selling it. I've currently got it on loan from him, and I think he's hoping I'll buy it from him, since I like to fix things. I don't have the cash to buy it now, though, even at half the going eBay rate for a fully functional model.
I may try to get his blessing to disassemble it far enough to check the contacts inside that switch, though.
