Home How-To Tip Of The Week Get On Balance - 1/21/08
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Sunday, January 20, 2008

Get On Balance - 1/21/08

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kidsI can think of a handful of Eureka moments in my life, but one of the most significant was the day in college when I finally understood how to control ambient and strobe exposures independently within the same shot. The gist is simple: shutter speed has no effect on strobe exposure. It took a while, but once I got it, a whole world of control opened up to me. Because a strobe's duration is so short-1/1000th of a second and faster-the entire flash occurs well within any shutter speed of that duration or slower. Think of it as a timeline: 1/125th of a second or 1/250th of a second doesn't matter-the flash happens just the same within each exposure. So the only way to modify the flash exposure-short of changing the flash's power or distance from the subject-is to open or close the lens aperture. In practice, it's easiest to start with the ambient exposure you want-say ƒ/8 at 1/125th of a second-and adjust the flash output until it matches the ƒ/8 mark, changing only the shutter speed then to adjust the ambient exposure. The best part of all of this is you can create great effects by slightly underexposing the ambience-say to make a sunset background even more vivid-and correctly exposing the flash by maintaining a constant aperture and simply adjusting the shutter speed. It will keep the flash exposure constant but can take the background from over- to underexposed in just a few clicks of the thumbwheel. Eureka!

Illustration: Timeline of Strobe vs. Shutter Speed


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