The proper setting for the f stop on a camera when taking a photograph can be determined based on the flashgun's guide number (GN), the distance to the subject, and the film speed being used.
flashgun's guide number (GN) =
= (f stop setting using ASA 100 film) * (distance to the object in meters)
f stop setting using ASA 100 film =
= (GN) / (distance to the object in meters)
f stop for film speed being used =
= (f stop for ASA 100 film) * (film speed factor)
Film Speed
Factor
ASA 50
0.7
ASA 100
1
ASA 200
1.4
ASA 400
2
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