By Matthew L Kees, guest to the MCP Actions Blog
Using Flash at a Distance (“up the down squares…”)
Flash-to-subject distance isn’t usually an issue indoors, unless you are bouncing the light off a high ceiling or you in a very BIG space, like a cathedral. Outdoors in open spaces, it can easily become a factor to your flash and camera settings.
A flash is built around a Xenon filled flash tube. The tube looks like a mini fluorescent light-bulb that is housed inside a reflector. The job of the reflector is to send the light in one direction. But, it also needs to spread it out some or it would only be lighting an area the size of the flash window.
When the light travels away from the flash it gets spread out in a rectangular shape. Both the height and width of the shape increases. As it spreads out it also drops in intensity. The intensity of the light falls off using what is known as the Inverse Square Law. In simple terms the Inverse Square Law means the light is very bright near the source but loses much of its intensity at a distance — using the formula, one over the distance squared.
The mathematics of the Inverse Square Law and why it is important:
With a subject ten feet away, the flash intensity falls to 1/100th of what it is one foot away. At 20 feet, it drops to 1/400th and at 40 feet the light falls to 1/1600th of its initial intensity. If you try to push it to 50 feet, your subject will only be getting 1/2500th of the light — one over fifty squared.
The same is true if you are bouncing the light off a 20 foot high ceiling. The total distance the light has to travel is at least 40 feet — up to the ceiling and back down to your subject. The subject(s) are getting less than 1/1600th of the flash’s intensity even if they are standing only three feet away.
Should you need to shoot with flash from far away, first open your aperture to let more flash light come in, and second increase the ISO for the same reason that raising the ISO requires less light when not using a flash.
If your subject is really far away, you might need to remove the flash from the hot shoe and use it in remote mode. Have someone hold the flash closer to your subject(s) or mount it nearby on a stand.
Note that ALL light falls off using this rule — flash, strobes, household lamps, even sunlight. But sunlight is so far away that another foot or two, or even many miles doesn’t factor into its brightness. The tilt of the Earth and its distance to the sun does however, change our seasons.