At high speed sychro mode,the GN of a flash decreases as the the shutter speed increases,why and how? I noticed that the highest sychro speed that many flashes can set is that of the slr,but duration time (eg.1/300s) of the flash is much less than the highest sychro speed (eg.1/90s)it can set with the slr,why? Is there a link between the flash duration time and the sychro speed? Thanks

-- legnum (legnum212@email.com), November 19, 2001

Answers

Take a look at the following URL:

http://www.photozone.de/flashtec.htm

-- Bill Boshoven (wboshoven@juno.com), November 20, 2001.

In normal synch mode, the flash must wait until the first curtain is fully open before it can fire, then the second curtain begins to close.. so the max synch speed is curtain travel time + flash duration.

in high speed synch mode, the flash pulses rapidly to create effectively a continuous light source for the duration of shutter travel. effective guide number then is controlled by shutter speed just as ambient light is controlled by shutter speed: faster speeds allow less light to reach the film.

rick :)=

-- Rick Oleson (rick_oleson@yahoo.com), November 21, 2001.

The fast pulses on their own could add up to the same intensity as the main flash burst, but the problem is that there's no time when both shutter curtains are fully open (if there were, there's be no need for high-speed sync) - so some of the light ends up on the film, and some ends up wasted when it hits the shutter curtains. That's why the GN drops as the shutter speed rises - as the shutter speed rises, the film is obstructed more and more by the shutter curtains, and therefore less and less of the light actually reaches the film.

-- Steve Dunn (steved@ussinc.com), November 21, 2001.