> What's a "Guide Number" and how does it work? For manual flash, you use the number to calculate the correct aperture for proper exposure. You divide the guide number by the number of feet (or meters use the correct unit) from flash to subject to arrive at the proper f stop. The guide number obviously varies by ISO.
In modern ttl flash photography, the guide number is mostly useful as an indication of the flashes maximum power. Tom Reese
