Nikon BC-7 Manuale di istruzioni - Pagina 20
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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.