High speed flash works differently to 'normal' speed flash.
Basically in 'high speed' mode the flash is on for a longer period of time, than in normal sync mode.
In normal sync mode, the flash fires either at the start (first curtain sync) or end (second curtain sync) of the exposure. So it tends to stop motion. With high speed sync, the flash outputs at a lower power setting for a longer period of time (yes, I know it sounds backwards). This continual light mode of the flash avoids synchronisation issues with the camera shutter. This also reduces the range of the flash, as it is lower power over a longer period of time, rather than one very bright flash.
Short story, long: High speed sync mode doesn't let you stop motion in the same way that normal flash sync does.
Message edited by author 2006-11-01 16:31:59.
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