Originally posted by blende8 There is not the flash duration for a flash.
The duration depends on the power it is emitting.
E.g. the AF540 needs 1/200 s to give its full power, but has the shortest possible time as 1/20.000 s.
no, in fact it needs a lot less than 1/200th
the 1/180 flash sync (5.4 mS) is the whole exposure time. this time is split into 3 segments. Leading curtain opening time, full open time, and trailing curtain closing time.
generally the two curtains move at the same speed, and each could move as slow as 1/180 of a second with the trailing curtain beginning to close the instant that the leading curtain gets fully open. therefore no part of the frame is exposed for more than 1/180 of a second.,
in reality the curtain opening and closing times will be shorter than 1/180, because they do need to leave time for the flash to fire in between.
I would suspect that they actually leave about 1-2 mS of full frame open time, and full power of the flash will be tuned to that time.
I tried to get this data once from pentax but they wouldn't give it.
The best way woud be to measure a very fast spinning object of known speed, and triggering the flash at each power, 1/64th etc and measure the angular rotation of the object to calibrate the flash. Full power is a little dangerous because duration may also depend on the state of charge of the capacitor and therefore be a little inconsistent.
some flashes have minimum power settings in the 1/20,000 to 1/50,000 range, but note that flash duration doubles for each doubling of power