Sigma 85mm 1.4 AF problem

Hey all,

I just ended day 2 of a 3-day shoot and I realised that my Sigma 85mm 1.4 on my 5D3 had some issues. It can't lock focus on anything whatsoever, even on one-shot mode. Basically I'm on back-button focus on camera, and every time I engage autofocus, no matter what AF mode, it will start to violently rack back and forth around that point as if it's hunting. In one section, I'll probably get 4 sets of "back and forth" focus rack motion.

Anyone faced anything similar before? it was just working fine yesterday, and nothing happened in between. No bumps, knocks, or kicks.

Same light and subjects? Many lenses will do that if they can't find a bright-enough subject with enough contrast.

No there isnt any low contrast target to confuse the AF. We're talking striped patterns and t shirts.

I think this has been an issue with some Sigma 85's. I would send it to Sigma, they'll fix it.

Try microadjusting it before you send it back. If it is way off that can make it hunt more.

It does sound like yours has a problem though, especially if it worked before.

It needs to be serviced and re-calibrated back to original specs, MFA won't really fix the issue.

Alright thanks guys I'll send it in tomorrow and get back to this thread.

Well that was a quick turnover. Cost me 160 SGD for parts and service, so about 125 USD. Decent.

Did it fix the problem

enginyr wrote in post #16879335
Did it fix the problem

Yup, it did as far as I can tell. Hopefully it's not a recurring issue. Some board replacement.