this lens is a little bit too sensitive – after a certain time the inner barral will have an increased backlash – which will cause a less accurate focusing.
the solution: You have to change the cam followers.
Here are the prices:
So, 10 USD for each piece. thera are 3+2. Plus post. Spareparts: 50 USD….. not too cheap.
If there would be all 3 resp 2 in the price – then would be OK.
But ithout this, You will have always focusing problems.
I will try to make this parts from PTFE