Thursday 3 July 2008

High-speed sync (FP) flash

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As I mentioned in my previous post, the EX-series of Speedlites have an option that allows you to use flash even when your shot requires a shutter speed faster than the X-sync (flash sync) speed. It's termed high speed sync mode (technically known as FP, or focal plane, mode for historical reasons, though this abbreviation has been referred to as fast pulse, which more accurately describes what the flash unit is doing) and is highlighted by a red box in the picture above. A quick remider - at shutter speeds faster than the X-sync speed only part of the sensor will be exposed to the scene at any one time, as one curtain starts to move before the other has come to rest. To overcome this, the unit pulses the light very quickly (50 KHz), effectively creating constant light for the duration of the shot - this is the elegant solution that FP mode offers. When I'm shooting outside I always have the flash unit set to FP. The price you pay is that the flash output is significantly reduced (to about a third), but by shooting at a wide aperture and getting close to your subject, less light output will be required as fill to balance the ambient, or key, light.

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