Quick Tip: Understanding ‘Flash Falloff'

Why you don’t always get the lighting you think you will

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Background 5 feet back
Flash can illuminate a brick wall that's close behind the subject. By moving the subject away from the wall, the flash falls off enough to leave the background in darkness (See Below).
Dan Bracaglia

If you’re ever at the Manhattan terminal of the Staten Island Ferry in the evening, you can catch a great free light show whenever a ferry approaches the dock: What looks like hundreds of fireflies flashing all over the boat. It’s the tourists trying to photograph the Manhattan skyline with on-camera flash.

Unfortunately, few (if any) of them will get decent pictures. What happens is that their flash “falls off”—it fails to reach far enough to illuminate distant objects. It’s not as if the light rays lose steam; those photons will keep going and going. But they spread out as they travel and thus become less concentrated.

The beam from a flash unit is cone-shaped. Photograph a relatively close object and most, if not all, of the light cone will cover it. But as you move the flash farther away, a smaller portion of the light cone hits the subject, as more of the light sprays wide of it. At a great enough distance, the flash’s light becomes imperceptible to the eye as well as to a sensor or film. Imagine trying to water a small plant at a great distance with a garden hose set to a wide spray—you may pump out gallons per minute, but very little of it will actually hit the plant.

That’s the bad news. The worse news: Flash falls off as the square of the distance. In plain lingo, this means that if you move twice as far away from your subject, you get only one-quarter the illumination on the subject. Triple your distance and you get only one-ninth the light.

Solutions? You can make the light cone narrower. If you have an accessory flash with a zoom head, setting the head to tele position will do this. Accessories’ such as the Better Beamer can also make a narrower cone. In either case, you still lose wide-angle coverage.

You can also set an accessory unit close to your subject and fire it from a distance via wireless trigger. Of course, this won’t work when shooting the skyline from the ferry. In that case, turn the flash off and crank up the ISO to take the shot with available light. If your camera has a night shooting mode, try that.

The principles, of course, apply not just to flash. They’re equally valid for continuous light sources as well, such as hot lights.

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