379
n
AGuide Number
To calculate the range of the flash at full power, divide the Guide Number
by the aperture.
For example, at ISO 100 the SB-800 has a Guide Number
of 38m or 125 ft. (35 mm zoom head position); its range at an aperture of
f/5.6 is 38÷5.6 or about 6.8 meters (or in feet, 125÷ 5.6=approximately
23ft. 7 in.).
For each twofold increase in ISO sensitivity, multiply the Guide
Number by the square root of two (approximately 1.4).