[joewehry]

I have the Elmar 90mm f/4 lens (chrome, long, not collapsible.)
If you are talking about the silver button on the camera by the side of the lens, do you mean the lens locking mechanism on the M ? That is for locking and unlocking the lens when you want to change it, not focusing it.

If that is not what you mean, but are wondering more on focusing, on my model, with the focus point set on infinity, that is the point which will be most in focus. (on my model, somewhere around 300+ feet) The balance of the picture (from the 20 ft up to infinity that you gave as an example) may be in acceptable focus only if the aperture is set to f/32 (on my model). If you have it set on a more open aperture, say f/5.6 with focus set to infinity, the area of acceptable focus may only be between 100-300 ft, not 20-300 ft.

If you "google" the phraes "download leica manual" you can sometimes find a pdf link with illustrations that can better explain with images what I'm talking about, and how to use the depth of field of a lens, the aperture and the rangefinder patch to more precisely focus.