Quote Originally Posted by Jim Jones View Post
Infinitely bright point light sources are rarely encountered in practical photography. Therefore, theoretical formulas beloved by some physicists have little places in a photo forum. Our light sources are modified by reflectors and lenses, and the environment also influences the amount of light on a subject. Get practical: get a light meter.
Get both!

One reason I switched from shoot-through's to reflective umbrellas for general use is that it controls spill and reflections better, so I can actually estimate flash exposure moving flashes around. Even if the 'perfect' light source isn't encountered in photography, it's important to understand how/why light behaves like it does.