[Logieberra (Logan)]

Correct me if I'm wrong, but if you point the scope at a light source (or anything nicely illuminated) with no camera or end adapters (unobstructed light path) you can measure the focal point (from a metal back surface) by simply casting the refracted light on a flat surface, like a wall. Just move scope away from wall until the object (e.g. a light fixture, whatever it points to) snaps into focus on the wall. There you have it. Crude, but works.