Expresses the strength of af flash. GN is (american) the light needed to expose the film/sensor correctly and is equal to the aperture (F-stop) to be used at ISO 100, at a distance of 1 foot.
European GN is the same but at a 1 m distance. Thus appr. 1/3 of the American GN (foot). GN 45 (European) means that you should use F.45 at a distance of 1 meter at ISO 100. This formula applies: GN/Distance = F-stop Example: GN 45/1 m = F.45 GN 45/2 m = F.22 (F.22 doubles the light twice, compared to F.45) In practiacal use, the flash light will be reduced by (distance x distance), when the distance to subject changes, meaning, that if you double the distance, the light will not be half, but 1/4. This is because the light forms are square, enlightened area (on the a wall, for instance). The area of a 1x1 square is 1/4 of the area of a 2x2 square. The aperure value system works so that F22 lets in 4 times as much light as F45. This way the aperture value system fits the logic of flashlight nicely. Jens Bladt Arkitekt MAA http://hjem.get2net.dk/bladt -----Oprindelig meddelelse----- Fra: Shel Belinkoff [mailto:[EMAIL PROTECTED] Sendt: 26. september 2005 21:04 Til: [email protected] Emne: Flash Question What's a "Guide Number" and how does it work? Shel
