I had a Takumar 50 F 4, very sharp, nothing bad to say, only reason I sold it was that I have a 50 1.7 and 1.4 and a couple of sets of extension tubes which I generally use with a 105 2.8 or 85 .18. I also have a Chinon 50 1.7 macro. It was made by Chinon and sold under both Alpa and Chinon in M42. Story is that was made to Alpa Kern Swiss sepcs for the Chinon Alpa M42 version and even used Kern glass. I have never been able to confirm this. I have used a Kern Swiss 50 macro and while the Chinon is good lens is not up Kern standards, and for that matter is not as good as the Takumar 50 F4. I need to have it repaired, a internal ring that the pin pushes down to stop the lens down is bend so the lens will stop down.
I had my 50 F 4 for over 20 years and other having to have cleaned an lubes due to disuse and living a desert, no issues with diaphragms sticking. On the other hand if you have a good 50 prime any reason not use extension tubes or a bellow?