Nikon SB-800 instruction manual ISO sensitivity factors, To calculate the correct aperture

ISO sensitivity factors

For sensitivities other than ISO 100, multiply the guide number by the factors shown in the table below.

ISO	25	50	100	200	400	800	1600
Factors	x0.5	x0.71	x1	x1.4	x2	x2.8	x4

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

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