The Guide Number formula is as follows: (GN)=distance (feet) Ă— aperture (f-number)
To find desired aperture or new distance divide the GN value on the exposure chart by either the desired distance or aperture. Main Light Power Setting (f5.6 LV - 1/4 power) a. Using the exposure chart, note that the power setting to achieve proper illumination for an aperture setting of f5.6 is listed as 1/4 power on the right-hand side of the exposure chart. b. That is, modifying the flash with a small umbrella or soft box, attenuates the light such that it will produce a correct exposure value at a distance of 5 feet from the subject, for an aperture of f5.6 at a power setting of 1/4. Note: The exposure chart is based on a flash unit with a Guide Number (GN) of 58 meters at a zoom setting of 105mm at full power (1/1). This is similar to the Nikon SB910 and the Canon 580 EX II and 600 RT EX models. For the purposes of this example formula, the exposure chart has been calculated for 70mm (straight flash) and 14mm (flash into umbrella)
MODEL: ELENA TSYKOVA WARDROBE: GEORGIE GIRL PHOTO: DAVID LLOYD
There are two sides of the chart: a. Straight Flash (left side of chart) b. Modified Flash (right side of chart) 59