Photographer Frank Donnino has come up with a dead simple way of figuring out how the catchlights will look in any photographic setup you could possibly imagine. If you’re having trouble visualizing how the incoming light will look on your subject’s eyes, just use a black marble to see where the reflections are coming from.
Just watch the brief video below, but it’s really a very intuitive idea. The shiny, opaque marble acts like a more dramatic version of an eye, easily showing where the lights are coming from. And then it’s just a matter of posing your subject properly—which is a whole different set of challenges.