Sorry, I forgot to put you in copy.
Chris Somers wrote: > These numbers can't be right -- especially that full power duration > is 1/150s. This number comes from the test of the recent Canon 550 EX flash. I remember a group of flashes were tested a while ago and numbers were usually around 1/200s to 1/300s for full power. > Many of Nikon's cameras give *full* sync at 1/250s > shutter speed. The duration of full-power flash HAS to be shorter > than that. It does not really have to. 90% of the flash power is still emitted in the 1/250s window. It is the flash trail which is partly blocked by the rear curtain. The result is just a possible slight underexposure compared to the official guide number. > I don't have my speedlight manual with me, but I thought that the > flash duration v. power tables were given in there. It all depends on how you measure the flash duration, as the flash intensity decreases as a negative exponential after the main burst. If you measure the mean burst width only, as most manufacturers seem to do, you only measure a duration which corresponds to about 70%(*) of the power. The magazine that performed the tests preferred a measure that corresponds to about 90%(*) of the power, hence the longer duration. This is not a major problem. As stated above, the only result is a slight diminution of the available guide number, when using fast sync speeds. Fast speeds are often used for fill-in, and brute flash power is rarely needed in these cases. Still, the problem worsen with bigger flashes, so care has to be taken before using huge strobes with a sync speed of 1/300s. Hope this helps, Nicolas. (*) I am nut sure of the exact numbers anymore, but the reasoning remains the same.
