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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