Nikon D300 User Manual (en)

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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=approximately 23 ft. 7 in.). For each twofold 
increase in ISO sensitivity, multiply the Guide Number by the square root 
of two (approximately 1.4).