Nikon D3X User Manual page 214

A
Guide 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 38 m or 125 ft.; its range at an aperture of f/5.6 is 38÷5.6 or about 6.8
meters (or in feet, 125÷5.6=23 ft. 7 in.). For each twofold increase in ISO
sensitivity, multiply the Guide Number by the square root of two
(approximately 1.4).
l
188