Nikon 25432 User Manual - Page 316

Note, however, that flash range will drop at shutter speeds between 1/250 and 1/320 s. The effective Guide Number varies with zoom angle as shown below. Zoom angle 14 mm 17 mm 24 mm 28 mm 35 mm 50 mm 70 mm 85 mm 105 mm Guide No. * 17/56 19/62 30/98 32/105 38/125 44/144 50/164 53/174 56/184 * ISO 200, m/ft., 20 °C (68 °F), SB-800 Calculating Flash Range D (maximum range) = GN (Guide Number) ÷ f-number (aperture) For example, at a zoom angle of 35 mm, an aperture of f/5.6, and an ISO sensitivity of ISO 200, the flash will have a maximum range of D = 38 ÷ 5.6 = 6.7 m (or in feet, D = 125 ÷ 5.6 = 22 ft. 3 in.). The minimum range can be read directly from the flash range bar graph or exposure meter. For each twofold increase in ISO sensitivity, multiply the Guide Number by the square root of two (approximately 1.4). 400 Multiply GN by 1.4 ISO sensitivity 800 1600 3200 2 2.8 4 U 290