On Sep 26, 2005, at 12:04 PM, Shel Belinkoff wrote:
What's a "Guide Number" and how does it work?
A Guide Number is a number based on flash output used to calculate
aperture setting per a given ISO and subject distance to obtain
correct exposure.
For example:
A flash has a Guide Number in feet of 80 for ISO 200. Divide 80 by
the number of feet to give the appropriate f/number for average
subjects in average sized room, so if your subject is 10 feet away,
you set the lens to f/8.
The *ist DS manual gives you GN and how to calculate aperture on page
149-150.
Godfrey
