On Thu, January 8, 2009 06:19, jonathan turner wrote: > Please can someone explain what Guide numbers are and what they do? A guide number is a measure of the light output of a flash, scaled to make it easy to calculate manual exposures for direct flash. The guide number depends on the flash power, the ISO, and the distance units (generally feet or meters). If the guide number for your flash at the power setting and ISO setting you're currently using is 100, and it's in feet, then the exposure at 10 feet will be 100/10 = f/10. At 20 feet, it'll be 100/20 = f/5. If on the other hand you want to use meters, and the guide number is 30 in meters, then at 3 meters the exposure will be 30/3 = f/10. For precise work people used flash meters (and polaroid tests) in the old days. These days people use test shots in digital, mostly. And most camera-mounted flashes have various kinds of automation that adjust the power to what they see coming back. But when I first used electronic flash, the guide number was all I had (a flash meter was too expensive, and too slow for news coverage). Even when I learned to do bounce flash, it was all I had. (The approach then was to estimate the distance flash-to-reflector and then reflector-to-subject and add them together, and then apply a correction factor for how reflective the reflector, usually the ceiling, was. And shoot B&W negative and work out the problems when printing.) -- David Dyer-Bennet, dd-b@xxxxxxxx; http://dd-b.net/ Snapshots: http://dd-b.net/dd-b/SnapshotAlbum/data/ Photos: http://dd-b.net/photography/gallery/ Dragaera: http://dragaera.info
