Joy Squared -- NEX-5N & Carl Zeiss Jena Flektogon 35mm f2.8 Exakta Mount
My experience with East German lenses is that most of them are optically pretty good, but their build quality leaves much to be desired; they don't age very well. I have gone through many of them and majority of them have some problems. Most common problem is very rough focusing, and the other is aperture issues. The rough focusing is due to the grease drying up. If you handle a Takumar lens, at about the same age, the difference is like, day and night.
This particular Carl Zeiss Jena Flektogon 35mm f2.8 (zebra verison) is no exception. The focusing is rough, and the aperture is starting to lose control. It's a very good lens, especially its amazing close focus capability, right down to 0.18 meters!