This lens has focus shift (so I heard). I own this lens too. I always assume that focus shift is mostly covered by DOF of the lens. This may reflect my experience. But, I am reading in some threads that there are some apertures in which the focus shift exceeds beyond DOF. I have not experienced this myself. I have only taken less than 100 pictures with this lens so far, and have not conducted scientific tests.
I really like this lens, and it's my favorite lens so far. My second favorite lens is Rollei 40mm Sonar. As you can see, I really like Sonar. I am asking this question because I want to make the best out of this lens. But, there's a lot of conflicting information floating on this forum. Maybe someone should make a sticky post clarifying all the FAQ's with this lens.