Guide number (ISO 100, m/ft)
Flash output |
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level | 14* | 24 | 28 | 35 | 50 | 70 | 85 |
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M1/1 | 14.0/45.9 | 26.0/85.3 | 28.0/91.9 | 30.0/98.4 | 36.0/118.1 | 38.0/124.7 | 40.0/131.2 |
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M1/2 | 9.9/32.5 | 18.4/60.4 | 19.8/65.0 | 21.2/69.6 | 25.5/83.7 | 26.9/88.3 | 28.3/92.8 |
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M1/4 | 7.0/23.0 | 13.0/42.7 | 14.0/45.9 | 15.0/49.2 | 18.0/59.1 | 19.0/62.3 | 20.0/65.6 |
M1/8 | 4.9/16.1 | 9.2/30.2 | 9.9/32.5 | 10.6/34.8 | 12.7/41.7 | 13.4/44.0 | 14.1/46.3 |
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M1/16 | 3.5/11.5 | 6.5/21.3 | 7.0/23.0 | 7.5/24.6 | 9.0/29.5 | 9.5/31.2 | 10.0/32.8 |
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M1/32 | 2.5/8.2 | 4.6/15.1 | 4.9/16.1 | 5.3/17.4 | 6.4/21.0 | 6.7/22.0 | 7.1/23.3 |
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M1/64 | 1.8/5.9 | 3.3/10.8 | 3.5/11.5 | 3.8/12.5 | 4.5/14.8 | 4.8/15.7 | 5.0/16.4 |
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*With the
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 |
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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
f/stop (aperture) = Guide number (GN at ISO 100; m/ft.) ⋅ ISO sensitivity factor ÷ Shooting distance (m/ft)
For example, for the ISO sensitivity of 400 (or ISO sensitivity factor: 2), with the
•Referring to the guide number table, determine the guide number as 28/91.9 (m/ft.). When the shooting distance is 3.5m (11.5 ft.), the correct aperture value is:
28/91.9 (GN at ISO 100, m/ft) ⋅ 2 (ISO sensitivity factor for ISO 400) ÷ 3.5/11.5 (shooting distance; m/ft.) = 16 (aperture)
•Set the aperture value 16 on both the
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) ⋅ 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
For example, for the ISO sensitivity of 400 (or ISO sensitivity factor: 2) and the shooting distance of 3.5m (11.5 ft) with an aperture of f/8:
3.5/11.5 (shooting distance; m/ft.) ⋅ 8 (aperture value)
÷ 2 (ISO sensitivity factor of ISO 400) = 14/45.9 (GN at ISO 100; m/ft.)
Detailed operation
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