Thanks to Godfrey and Paul for explaining the flash terms. Time to take a few aspirin <LOL>
Shel > [Original Message] > From: Godfrey DiGiorgi > > GN == Guide Number ... A number that allows you to calculate the > correct aperture for proper exposure using flash at a given distance > and ISO sensitivity. For instance, if a flash has a GN (in feet) of 50 > with ISO 200 film, it has the full power enabling you to set f/5.6 for > photographing groups at 8-9' distance. > > GN is often used as a power rating, usually quoted at ISO 100 in feet > and meters. > --- > > P-TTL == Preflash-Through The Lens flash metering ... TTL flash > metering says that there is a sensor which watches the film/sensor > plane *during* the actual exposure and sends a signal to a compatible > flash unit when enough light has reached the sensor to quench the > output. P-TTL goes beyond that: it makes a flash prior to the actual > exposure and combines the ambient TTL metering with the flash to arrive > at the proper exposure, then makes the actual flash and exposure > afterwards.
