I have a Rodenstock Rodagon 50mm f2.8 that I acquired second hand a while ago. Unfortunately the aperture ring does not move smoothly. It is very stiff, so stiff in fact that the lens tends to come unscrewed from the lens board when changing aperture. It is possible to disable the click via a slider but even then the aperture ring is stiff but much less so than with the click enabled. Does anyone know how I can access the click? It seems that the problem is with the aperture ring mechanism rather than the aperture itself. It may be gummed up with old grease and dirt. I don't want to disassemble the lens too far as I will probably ruin it but at the moment it is unusable and probably not worth professional repair so I suppose I don't have a lot to lose. It looks like the plastic parts, including the aperture ring, form a collar around the metal cylindrical core that contains the lenses. There is a plastic locking piece let into the top surface that might come out with a drift and release the plastic collar without any need to remove lens elements. I am reluctant to investigate further without knowing what I am doing. Incidentally there are some odd markings on the lens "AR 400-820 nM" which seems to refer to light wavelength.