Samsung ER-GX1LZBAA - Manual (Page 153)

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Using the Built-in Flash
ƈ The following equation calculates the aperture value for shooting distances.
Aperture Value Used F = Guide number ÷ Shooting distance
When sensitivity is [ISO 200] and shooting distance is 5.2 m, aperture value is:
F = 15.6 ÷ 5.2 = 3
If the resulting number (3, in the above example) is not available as a lens aperture, the smaller
number that is closest (2.8, in the above example) is generally used.
Calculating Aperture Value from Shooting Distance