Assuming a typical white-light focusing flashlight (e.g, Mag-Lite) with a parabolic reflector focused to produce a convergent/divergent beam, how do I calculate the smallest achievable spot size (i.e, smallest beam cross sectional area) at a given distance? How does this vary with reflector diameter, source size, focal length, distance to target, and beam focus (whether source is inside/outside the focal point)? I can't just use angular field = arctan (source dia. / focal length), as the beam can be initially convergent then divergent. Observationally it seems to involve mirror diameter since a large WWII searchlight beam converges for a long distance before it diverges. Looking for the right formula.