I noticed here that this 180 mm lens does not handle extension well. I have the 180 mm 2.8 AFD Nikkor lens. Birna said years ago it handled extension tubes really well. It may not be totally Apochromatic but that one ED glass of element seems to handle chromatic aberration pretty well. I know I’ve used it with a 6T close-up lens and that works very well, but I’ve never tried it with extension tubes. I will have to try it when I get home. I will keep you posted. Regards, Gerry.
Interesting: Since the Voigtländer 180/4 doesn't seem to have any CRC, and just has a _really_ long helicoid, I'm surprised it would care about extension tubes: 1:4 (closest focus) should be much the same as 1:2 (with a PN-11 or whatever). Will have to try mine out with the PN-11 and see.