Nikon Speedlite SB-800 - Manual (Page 49)

43
To calculate the correct aperture
Calculate the correct aperture by using this equation and the guide number table,
according to the ISO sensitivity, flash output level and zoom-head position that are set:
f/stop (aperture) = Guide number (GN) x ISO sensitivity factor ÷ Shooting distance (m/ft)
• Set the same aperture on both the SB-800 and the camera or lens.
To calculate the guide number
Calculate the guide number by using this equation, according to the shooting
distance and aperture required.
Guide number (GN) = Shooting distance (m/ft) x Aperture ÷ ISO sensitivity factor
• Referring to the guide number table, determine an appropriate flash output level
corresponding to the guide number obtained above, then set the same value on
the SB-800.
• In Distance-priority manual flash operation, the guide number (indicating the amount of
light generated by the flash) is automatically determined by the SB-800 according to
the distance value and aperture set.
ISO sensitivity factors
For sensitivities other than ISO 100, multiply the guide number by the factors
shown in the table below.
Detailed oper
ation
ISO 25 50 100 200 400 800 1600
Factors x0.5 x0.71 x1 x1.4 x2 x2.8 x4