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The term “flash falloff” applies to several different phenomena. One is corner falloff, where the corners of the picture are distinctly darker in flash shots only, the result of a mismatch between a wide-angle lens and a not-so-wide flash beam.
Falloff also refers to the weakening of the flash beam as it travels a greater distance—what we’re here to explain. If you’ve done any serious picture taking, you know that your on-camera flash can’t possibly illuminate the Statue of Liberty from the middle of New York Harbor. The flash “falls off” over distance according to a mathematical principle: It falls off inversely as the square of the distance. Move twice as far away, and you get only one-quarter of the original power.
But why? Don’t the photons fired by your flash travel forever? Are they bumping into something on the way? And why do they dissipate so fast?
Visualize the flash beam as a cone (it’s a squared-off kind of cone, but still conical). The cone starts narrow at the flash head, then widens out. The photons that spew out of the flash head in a concentrated bunch spread out over a wider area the farther they travel. How much? Well, the cross section of a cone is a circle. If you double the diameter of a circle, its area quadruples. But wait—it’s still the same number of photons covering that bigger area. So if you have, say, a human face in that flash beam, it will be lit by only a fraction of the photons if it’s located in a wider spread of the light cone.
By now you’re saying, why not make the cone of light narrower to go farther? That’s exactly what zooming flash heads do: They concentrate the beam for longer focal lengths, sacrificing wide coverage for increased reach (increasing corner falloff, by the way, if you don’t go to a longer lens). For even more extreme narrowing of the cone, you can get add-on Fresnel lens gadgets like the Better Beamer.
But the light will still, eventually, “fall off,” and you’ll be left with the alternative of natural light. Sorry, but them’s physics.
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