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ISO sensitivity factors
For sensitivities other than ISO 100, multiply the guide number by the factors shown in the
table below.
To calculate the correct aperture
Calculate the correct aperture by using this equation and the guide number table,
according to the ISO sensitivity, shooting distance, flash output level and zoom-head
position that are set:
f/stop (aperture) = Guide number (GN at ISO 100; m/ft.) x ISO sensitivity
factor ÷ Shooting distance (m/ft)
For example, for the ISO sensitivity of 400 (or ISO sensitivity factor: 2), with the zoom-head
position adjusted to 28mm, and the flash output level set to M1/1:
• Referring to the guide number table, determine the guide number as 32/105 (m/ft.).
When the shooting distance is 4m (13.1 ft.), the correct aperture value is:
32/105 (GN at ISO 100, m/ft) x 2 (ISO sensitivity factor for ISO 400) ÷ 4/13.1
(shooting distance; m/ft.) = 16 (aperture)
• Set the aperture value 16 on both the SB-800 and the camera or lens.
To obtain the correct flash output level
Calculate the guide number (GN at ISO 100; m/ft.) by using this equation, according to the
shooting distance and aperture required.
Guide number (GN at ISO 100; m/ft.) = Shooting distance (m/ft) x Aperture (f/) ÷ 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.
For example, for the ISO sensitivity of 400 (or ISO sensitivity factor: 2) and the shooting
distance of 4m (13.1 ft) with an aperture of f/8:
4/13.1 (shooting distance; m/ft.) x 8 (aperture value) ÷ 2 (ISO sensitivity factor of
ISO 400) = 16/52 (GN at ISO 100; m/ft.)
• When the zoom-head position is adjusted to 28mm, set M1/4 flash output level on the
SB-800, which can be obtained from GN 16/52 (m/ft.) corresponding to the zoom-head
position of 28mm referring to the guide number table.
• In Distance-priority manual flash operation, the correct flash output level is automatically
determined by the SB-800 according to the shooting distance, aperture and ISO
sensitivity that are set.
To calculate the shooting distance
Calculate the shooting distance (m/ft.) by using this equation and the guide number table,
according to the ISO sensitivity, aperture value, flash output level and zoom-head position
that are set:
Shooting distance (m/ft.) = Guide number (GN at ISO 100; m/ft.) x ISO sensitivity
factor ÷ Aperture (f/)
Detailed operation
ISO 25 50 100 200 400 800 1600
Factors x0.5 x0.71 x1 x1.4 x2 x2.8 x4