The essence of a focal plane shutter is that two shutter curtains travel over the film plane. For longer exposures (longer or equal the flash sync speed) the first curtain opens the 'window' and lets the exposure begin whereas the second curtain closes it again. For these longer exposures the (electronic) flash has to be synchronised such that it emits its light between these two actions - this is the time when the 'window' is fully open. For shorter exposures the second curtain begins closing the window on one side of the window when the first curtain has not even opened it completely at the other end - effectively a 'slot' is created (hence the German name Schlitzverschlu� - slot shutter). The timing of the two curtains following each other now determines the width of the slot and thus the actual exposure that the film gets. As for these shorter exposures there is no time when the complete film window is free, a standard electronic flash burst would result in only the part of the film being exposed that the 'slot' leaves free (and the rest being dark).
(I have always been amazed by the fact that a focal plane shutter this way does not capture 'a moment in time' but rather a certain duration in one image...) The trick with High Speed Sync now is that the flash emits a series of smaller flash bursts that last as long as the 'slot' travels over the film plane. As this distributes the total energy that the flash can emit over time and as now the slot width determines the amount of light that the film gets, the GN becomes smaller. Sven Zitat von Tanya Mayer Photography <[EMAIL PROTECTED]>: > Nenad, > > that makes sense to me - if the shutter speed is faster then less light > would be getting in, thus the flash would be less effective? I think.... > > fairy. > > ----- Original Message ----- > From: "Nenad Djurdjevic" <[EMAIL PROTECTED]> > To: <[EMAIL PROTECTED]> > Sent: Thursday, February 19, 2004 4:36 PM > Subject: AF360FGZ high speed sync > > > > My flash manual says that as the speed gets higher the flash guide number > > becomes smaller. This is counterintuitive as I would have thought it was > > the other way around. Anyone care to explain? > > > > >
