At speeds higher than the synchro speed the flash is required to emit multiple light bursts, hence the more bursts the less power for each each burst and smaller effective guide number.
Servus, Alin Nenad wrote: ND> My flash manual says that as the speed gets higher the flash guide number ND> becomes smaller. This is counterintuitive as I would have thought it was ND> the other way around. Anyone care to explain?
