20 November 2008

Better depth of field control with flash and high speed sync

With Canon's cameras and add on Speedlite flashes you usually have the chance to set high speed sync or focal plane (FP) mode to let you shoot at shutter speeds faster than the cameras normal sync speed. Why not enable it all the time?

The idea of high speed sync is that the flash fires several times rather than just at the start or end of the exposure. It needs to do this as the shutter only uncovers part of the sensor at a time during the faster than sync speed shutter speeds. Think of it as a moving slit passing over the image, since that is actually what's happening.

With high speed sync enabled you can shoot at 1/500s at f/2 for depth of field control and have the flash fire to fill-in the shadows. The downside is that you lose a bit of flash power as it needs to fire several times during the exposure. In fact the faster the shutter speed the smaller the slit making the exposure and the more times it needs to fire during the exposure; so the less power.

Most of the time you need this faster shutter speed it's because you, the photographer is using a wide open aperture so the lower flash power is actually less of a problem than it might seem.

High speed sync is available on the Speedlite 550EX, 580EX, 580EX II, 420Ex, 430EX, 430EX II, Macro Ringlite MR-14EX and Macro Twinlite MT-24-EX.


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