Nikon BC-7 Instructions Manual page 20

20
EXPOSURE DETERMINATION BY GUIDE NUMBERS
The maker of each type flash bulb publishes the guide numbers, which
are conveniently used to determine the correct exposure. Divide the
guide number ( GN) by the subject distance (D) or the f.number (A) ,
and you will be given the f·number or the subject distance to be set,
respectively.
GN
D or A A or D
However, since such guide numbers are generally given on the basis
of a fixed· type, large.diameter reflector with a polished reflecting
surface, and not of a fan.fold, wide. angle type reflector with a light
scattering reflecting surface, such as of the BC-7 flash unit, it is nec·
essary to increase the exposure by opening the aperture about a half
stop larger than the value obtained by the above calculation.
Example 1.
Flash bulb :
Shutter speed:
Speed of film :
Guide number:
Subject Distance :
200 =13 3
15 .
G.E. M2 or Sylvania M2
1/ 30 sec.
( Turn the synch selector dial on
the Nikon F camera until a white dot and F
appear in the window.)
ASA 125 •
About 200
15 ft.
Then, open the aperture of lens a half stop.
larger than f/13.3, that is, about fill.