help: how to change beam angle using reflector and lens instead of rotating head

Can someone with the time and knowledge give me a brief physics lesson? I would like to learn how to adjust the beam angle of a flashlight in much the same way a rotating head would do however with a more limited range of motion (say 20 degrees from horizontal). I know that car headlights do this - is it by angling the entire reflector? This is for a stationary horizontal flashlight meant to light the path ahead without blinding oncoming passersby. Unfortunately this all needs to fit in a housing about the diameter of a AA battery. I know, I know, it's a lot to ask, but any help would be GREATLY appreciated and I would love to share the results with you all.

I should not: a very simple, albeit inefficient, idea i had is to make a sort of adjustable visor like a knight's helm with a flip up face guard to go over the front of the light. I'm going to cut some tubing today for a mock-up. I know this must not be a very good solution, but I'm going to give it a whirl as it's probably the cheapest and easiest method of angling the beam downward.