Hi Klaus,
How can I modify my 50 f1 to focus a bit further away? I assume one has to move the rear elements closer to the front elements...
A skilled mechanic has to take the rear lens part out, shorten and modify the barrel, then set the rear lens part in again, either glued or screwed.
I have a guy here in Germany who does that... (only works if lens is free of dents especially on rear part, only one of my four worked)