Adam Maas <[EMAIL PROTECTED]> wrote: >Shel Belinkoff wrote: >> What's a "Guide Number" and how does it work? > >The Guide number is flash distance times aperture. > >So a guide number of 30 feet will give a flash working distance of 15' >at f2, at f4 it would be 7.5'. Meters work the same way, just in meters. > >Minimum distance is 1/5 of maximum distance, so minimum distance in the >above example would be 3', generally there's always a minimum coverage >distance as well (usually around 2') which is inherent to the flash itself.
Note that you have to specify an ISO number for the guide number to be meaningful. ISO 100 is usually used. And you'll want to know the angle of coverage (or lens focal length). As you zoom out the flash head (to cover wide angle lenses) the guide number goes down. Obviously, for fixed head (non-zoomable) flashes this can be ignored. -- Mark Roberts Photography and writing www.robertstech.com
