When I play with M12 lenses (board lenses. Fixed fl. Aperture) , I can focus them to a distance and they have a certain depth of field. Objects within the Delta of field looks sharp. Can these lenses be focused to infinity ? And if that is possible how can I do that ?
The "marked" focal length specification of a lens is measured when focused at infinity.
So focal length may be "defined" at infinity (a fixed reference), but in reality, it is the distance behind the lens to the current focused point. This focal length (distance behind the lens) changes with distance focused in front of the lens.
To focus closer than infinity, the lens (or the front element of the lens) is moved slightly forward, further forward than what infinity requires.
So, if a problem focusing at infinity, it would seem all you have to do is to move the lens slightly forward. Adjust the focus ring so the front element is fully extended (set to the infinity mark), then adjust lens position so that it does focus at infinity then. This is the proper lens mounting distance.
Details: The focal length is NOT simply the distance behind the physical lens. It is measured from a special focal node design point which is often somewhere well inside the lens. In wide angle lenses, this node is well behind the actual lens. In telephoto lenses, it is slightly in front of the front element. In more regular lenses, it is well inside the lens somewhere.