Hi

I have the above lens which has been serving me well. Of late, I
notice that there is some problem when I turn the f-stop ring. When
turning to apertures from f5.6 to f2, the turning was smooth and
normal. When I start turning them from f8 to f16, it gets tigher and
tigher. At times, the turning is so tight that it unscrews the lens
slightly from the camera body and in other instances, the turning
changes the lens barrel to "collapsible mode".

What is the problem here? Is there a way to fix this at home, or do I
need to send it to a Leica technician to have it fixed? How much would
it roughly cost?

Thanks!