Hi, not sure if you still have this issue. I have just decided to look into it again, as I had a lot of focus problems with my Sigma 70-200mm HSM lens. Since there is no Sigma support here in Beijing, I took a look - and fixed it.
Here is what I did: On the front of the lens barrel is the rubberised focus ring. This can be removed (gently ply it away from the barrel by sticking a flat bit (or plastic letter opener) between the barrel and the rubber grip. It will come not entirely loose, so you need to work your way around the lens. Keep doing that until you moved it about half way forward. At this point two fixing screws become accessible on the barrel below.
In my case, one of the screws had come loose and that blocked the focus ring from turning, as the screw was blocking rotation in the access hole on the barrel.
Hope this helps others. Simple to fix, if you know how.
SOURCE: My Sigma APO 70-200mm f/2.8 HSM lens functions
Am having the same problem. I fixed it once but can't remember how I did it. I think there are screws under the mounting ring -which has four screws itself. Then, I think under the green computer board, there are other screws that should be tightened... I will look at mine tomorrow and come back here to report.
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