Pentax SLR Operating Manual page 152

150
Calculating Aperture Value from Shooting Distance
The following equation calculates the aperture value for shooting
distances.
Aperture Value Used F = Guide number ÷ shooting distance
When sensitivity is [ISO200] 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.
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