Guide number – Nikon SB 20 User Manual
Page 45
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Guide Number
See the table at right for guide numbers of the film
speeds and zoom settings.
In M mode, use the following equation to determine
aperture:
guide number
f/stop =------------------------------------
flash-to-subject distance
With ISO 400 film and the zoom set knob at N, the
SB-20’s guide number is 60(m) for full output. If the
subject is 15 m away, divide 60 by 15 to get f/4.
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