When I got this lens, it refused to focus to infinity, 15 meters the most, especially when using the widest aperture. At f/6 or more the problem disappeared, most likely due to the wider DOF. So I shot mainly short focus for a while, since everything else is great with this lens. Today I decided that it is time to 'cure' this illness, so I conducted a small operation. I found that there is a spacer ring between the mount and the lens. I tried to remove the ring and the lens focused past infinity. Well, this is better than before, but the problem is the aperture ring stuck because it was making a direct contact to the camera body. The spacer must stay. So, the remedy involved two hours of sandpaper work, trimming the ring to more than half of the original thickness. The results: infinity focus and a bunch of sore fingers