I bought a used sigma 50mm 1.4 from Amazon recently with shaky barrel. I tried the lens, focus perfect but the shaky barrel is annoying. So I manage to fix the barrel. During which I need to dissemble the back part of the lens, no optics is touched at all. And after the fix although the barrel is solid as a rock, I get front focus about 2-4 inches. That's even more annoying than the barrel!!! I then set up to fix this problem. I read some post that you can add space between the lens and the metal mount to fix the front focusing. Tried it, didn't work. I struggled a lot, read articles about phase detection mechanism on the camera and finally I found that it's not the space that matters, it's the mounting angle between the lens and the camera!!! Let me explain the problem and how I fix it in the following. Following is just my theory. I need more test from the other sigma users to test it.
What do I mean the mounting angle here? The lens body is not made as a whole, and normally there is a metal mount which is fixed by 4 screws to connect the camera. In principle the lens should be perpendicular to the senser surface. But because the mount is fixed by screws, there could be a small angle bias from the perfect position. And I think that matters, a lot, on focusing. Interestingly that only matters for those cameras using phase detection focusing. I guess someone must tried to focus the lossy lens with livemode and found that works perfectly, no mis-focus at all. That's because livemode focusing is using contrast detection which use the information on the senser directly. Why phase detection is troublesome? Because it don't use the information on the senser, instead it uses the focus screens on both side of the senser. by compare the information on two screens, the body gives instruction to the lens, only once...... the lens responds and finishes the focusing process. This is a tricky process. If the lens is in a wrong angle, one can imaging that there could be a shift for the two images on the focus screen. the body will misinterperate it and send wrong instruction. The above are based on what I learned and just my theory. I am more than welcome to any comments and corrections.
Then how do we fix it? Some one suggested to change the space between the mount with the lens body by adding/removing insert in between. I tried it with no success. I also found some other people use the method with different outcome. Some claimed and problem solved well others found no help. When I think about it now I figure maybe in the successful casees the lens happened to find the correct angle between the mount and body. So what need to do is not to change the distance, but to fine tune the lens-body angle. How do we know which way (screws) to change? The debugging idea comes from my wife. There are more than one focus point on the modern DSLRs. choose the center focus point, take one picture. And choose four points surrounding the center, take 4 more pics. On all the five pics, I belive you will find some of them front focus well others back focus. If you see a far focus point, loosen the screw on the correspond direction on the lens. if you see a close focus one, tighten the screw. Only little bit when you do this, you don't want to have a shakky lens anyway. And you also want to lose all the screws little bit at the beginning to leave space for operation. Repeat the process until you have good focus. This works on both my 50 1.4 and 24 1.8. My 24 1.8 needed more than +20 on my 7d which really drive me crazy because I am not the original owner. But after these simple adjustment it works on both my 5d and 7d, without any MA.
So try it and let me know how it works. Help me to test my theory and help youself
Sorry I am lazy, so no pics, no links. writing this on is already not my normal state