Monday, February 13, 2012
Three Secrets To Flash Success—02/13/12
The three things every photographer should know how to do with a flash
|This Article Features Photo Zoom|
1. Balance the flash exposure with ambient.
Apertures (f-stops) affect both ambient and flash exposure, but shutter speeds only change the ambient. This is because the flash duration is so fast, it's faster than the fastest shutter speed you have. Once you get your head around this concept you can actually control these two exposures independently within the same shot. For instance, on a normal blue sky day in which the exposure is 1/100th at ƒ/11, and you've set your flash to manually illuminate at half power, if you adjust the aperture to ƒ/16 both the ambient and the flash will be darker—one stop underexposed, in fact. But, if you were to leave the aperture at ƒ/11 and adjust the shutter speed to 1/200th, the flash exposed foreground subject would look the same, but the ambient background (say, a blue sky on a sunny day) would be darker—one stop underexposed. With this very simple principle in hand and a very basic technique, you can do a whole lot to a picture by separating the flash exposure from the ambient. This is a technique that is used in every kind of flash-based photography and at every level, from snapshots to magazine covers. And it all starts with the flash on your camera.
2. Your flash works better when it's not on your camera.
Well where else is it going to be, you might be asking. In your hand. Or on a stand. Literally anywhere, as long as it's not directly above the camera affixed to the hotshoe. Why is this so important? Because frontal light—especially from an unmodified strobe—is very harsh, and unflattering, and shadowless. Shadows actually help define shape and give pictures dimension. Full frontal flash… well it just rarely looks good. Simply moving the flash to arm's length from the camera (by holding it in your outstretched and slightly raised arm, tethered to the camera via a dumb PC cord or a smart dedicated TTL cable—the latter of which maintains through-the-lens metering and auto-output functionality) adds shape, is more flattering to faces, and generally looks worlds different—and much better—than the “bucket of light” look that will come from a flash on the camera.
Whether you prefer TTL auto mode (which I use sometimes) or fully manual flash output (which I also use sometimes), the important point is that you are deciding and you are controlling the flash, as opposed to flipping it on and hoping it works. So here's how to control TTL auto output: use the flash exposure compensation controls (indicated by a lightning bolt next to a +/-) to tell the flash you want to overexpose or underexpose. If your images are coming out too bright and hot, dial it down to -1/3 or -2/3. Too dark, boost it up a bit to +1/3. And if you can keep in mind that a flash assumes everything should look middle gray, you'll know to tell it to underexpose an already dark scene (a black cat in a black room, for instance, won't reflect as much light as a “normal” scene and so the flash will up the power and overexpose the result) and overexpose an already light and bright one.
If you really want flash control, though, consider mastering manual mode. Here's all you need to know for controlling the brightness of a flash: change its distance, change its power, or change the aperture. There's actually an equation for this: ƒ/ = GN over D, which means that if you know the guide number (the power rating of a flash) and the distance to the subject (from the flash, not from the camera) then you can calculate the appropriate aperture. By re-jiggering the formula, you can calculate any of the three items if you know the other two. Even if you're not interested in the formula, the principle comes in handy. With an aperture set, you can adjust the flash exposure in two ways: move the flash, or adjust the power. With a distance set, adjust the power or the aperture. With the power set, adjust the aperture or distance. That's all you need for full manual flash control.