Guide Number - Nikon SB 20 Instruction Manual

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
flstop
= - - : : : . - - - - - - -
flash-to-subject distance
With ISO 400 film and the zoom set knob at N, the
S8-20's guide number is 60(m) for full output. If the
subject is 15m away, divide 60 by 15 to get f/4.
32
Light output
Zoom
setting
W
Full
N
T
w
1/2 N
T
w
1/4
N
T
w
1/8
N
T
w
1/16
N
T
1600
800
88 62
(289) (203)
120 85
(394) (279)
144 102
(472) (335)
62
44
(203) (144)
85
60
(279) (197)
102 72
(335) (236)
44
31
(144) (102)
60 42
(197) (138)
72 51
(236) (167)
31 22
(102) (72)
42 30
(138) (98)
51 36
(167) (118)
22
15
(72)
(49)
30 21
(98) (69)
36 25
(118) (82)
Unit · m( ft)
ISO film speed
400
200
100
50
25
44 31 22 15
11
(144) (1U2)
(72) (49)
(36)
60
42
30
21 15
(197) (138) (98) (69) (49)
72 51 36 25
18
(236) (167) (118) (82) (59)
31 22 15 11 7.8
(102) (72)
(49) (36) (25)
42
30
21 15 11
(138) (98) (69) (49) (36)
51 36 25 18 13
(167) (118) (82) (59) (43)
22 15
11
7.8 5.5
(72)
(49) (36) (25) (18)
30 21 15 11 7.5
(98) (69) (49) (36) (25)
36 25 18 13 9
(118) (82) (59) (43) (30)
15 11 7.8 5.5 3.9
(49) (36) (25) (18) (13)
21 15 11 7.5
5.3
(69) (49)
(36) (25) (17)
25 18 13 9 6.3
(82)
(59)
(43) (30) (21)
11 7.8 5.5 3.9 2.7
(36) (25) (18) (13) (88)
15 11 7.5 5.3 3.7
(49) (36)
(25) (17) (12)
18
13 9 6.3
4.5
(59)
(43) (30) (21) (15)