The shift is most likely caused by a change in distance between the front and the rear element of the lens. Might be caused by opening the lens to clean the inside and reassembling it a little different as it was before. As long as you are aware of this it is no big issue. You will get sharp prints as long as you focus a second time after stopping down.
Maybe the shift can be corrected by rearranging the elements. But if you are not experienced in this it might be better to leave it as it is.
Last edited by Critt VanCranendonck; 02-12-2013 at 03:58 AM. Click to view previous post history.