Originally posted by yozza fast flash duration and a high output of light.
With small AA powered flash units high speed* and high light** output are mutually exclusive. If you want high power get a studio monobloc flash, at full power [1:1] their T.1. times are faster than any IGBT flash. IGBT flash units are slow as hell at 1:1 power***, they are only faster at low output levels where the IGBT switch truncates the output of the flash, leading to shorter durations.
Also it is possible to get colour inaccuracies with short flash duration due to the oscillations in the plasma arc with short pulses. Flash units can vary as much as 100 kelvin per power level. If you want to do fashion photography you need a flash that has low colour variance, or a really good set of flash gels - though gels limit the type of modifiers you can use with your flash.
* short T.1. times, some manufacturers only state the T0.5 spec, or hype how fast their flash is at this spec, they are moving the goal posts. When it comes to stopping motion, a short T.1. time is all that really counts.
** usually measured in Watt seconds, sometimes Joules.
*** the first generation AF540FGZ T.1 isn't much faster than 1/180th at 1:1