In article <316B7F4E.41C67EA6@cs.tu-berlin.de>, stede@cs.tu-berlin.de says...

>I just bought a used Micro Nikkor 55/2.8 (MF) and am
>considering to look for a PN-11 extension to get from
>1:2 to 1:1. Do people have experiences with the
>optical performance of this combination? Any comments
>will be appreciated.

A PN-11 is intended to get the 105mm macro to 1:1 - it will get the
55mm much closer, but you will run into the problem of finding the
focus possibly inside the front end of the lens! The PK-13 is the 1:1 tube for the 55mm. Tubes on the 55mm (unless well stopped down) are a bit problematical because the floating element compensations will not
be optimised with a tube installed (though at minimum set focus, it
will be close enough). I have never been very happy with this lens on
a 2X converter as an alternative to tubes (stopping down helps), but that is an option. The 55mm is a superb normal lens that goes to
1/2 life size with excellent performance (the 60mm Micro Nikkor is
a superb macro lens that focuses to infinity with some compromises),
but beyond that, performance is compromised with the 55mm, though
still useable.
Hope This Helps