Ray, by "lens" you mean the objective? That's the lens that these 50mm finders are focused by. The the objective is loosened and the locking ring is then adjusted to focus. The lens is then retightened to fix the focus.
I remember a little instruction leaflet on this, I just can't remember where I saw it.
I found
this set of 'instructions' on astronomyforum.net. It shows what I mean.
Mental.