Hi Roger,
thank you for your reply.
Maybe we are talking about different types of locks ?
I have come across three different types, so far:
a) a round knob as found on older Elmars, the 50/2 Summicron v2 ("rigid"), the Canon 50/1.5, the Nikkor 50/1.4, etc. This is the one I referred to. It does help me focusing.
b) a spur as in the 35/2.8 Summaron that I once had. Is the 35/2 v1 similar ?
c) a flat "button" as in the Canon 50/1.2
I was referring to a) and just imagined, that when the lens is locked at infinity, and
manipulated with the camera, the lock gives the focusing helix additional stability,
since it fixes the lens' movable part to the lens mount.
For me, b) was a pain, and c) somehow too, since it didn't help me focusing in any way.
Best,
Roland.