To find the correct aperture to use, divide the guide number by the distance between the flash and the subject. For
instance, suppose you find that the guide number of the bulb with the film in use is 160. If you then want to take a
photograph at a distance of 10 ft. from the subject, divide 160/10 = 16. Therefore, the correct aperture to use is
f16. Alternatively, if you want to use an aperture of f8 for any reason, then the correct flash distance is 160/8 = 20.
So the flash must be 20 ft. from the subject.
APERTURES WITH CLEAR FLASH BULBS
Distance
G.E.,
Mazda,
G.E.C. No. 1
Philips:
PF 1
AG1
Mazda,
G.E.,
G.E.C.:
No. 5
Philips:
PF 5
Sylvania:
Press 25
Philips:
PF38
Sylvania:
Press 40
G.E.:
No. 11
Philips:
PF 60
G.E.:
No. 22
Sylvania:
No. 2
6 ft. (2m.)
16
--
--
--
8 ft (2.5m.)
12.5
--
--
--
10 ft (3m.)
10
16
--
--
12 ft. (3.6m.)
8
16
16
--
15 ft. (4.5m.)
6.3
11
12.5
16
20 ft. (6m.)
4.5
8
10
12.5
25 ft. (7.5m.)
4
6.3
8
11
30 ft. (9m.)
3.5
5.6
6.3
9
In bright rooms (kitchen, bathroom) or with films faster than 100 ASA, use next
smaller aperture. In very large rooms, at night outdoors or with slower 40-64 ASA
film use next larger aperture.
APERTURES WITH BLUE FLASH BULBS
(For 32 ASA Daylight Type Reversal Colour Film)
Distance
PF 1/B
No. 1B
AG 1B
PF 60/97
No. 22B
3½ ft. (1m.)
f
16
--
5 ft. (1.5m.)
f
11
f
22
7 ft. (2.2m.)
f
8
f
16
10 ft. (3m.)
f
5.6
f
11
CONVERSION OF FEET AND INCHES INTO METRIC UNITS
Many cameras are marked only in either the metric or British system, while
most of the tables in this book are also given only in one system. The table
below shows at a glance equivalent lengths.
British to Metric
Metric to British
1/8 in.
0.32 cm.
0.5 cm.
3/16 in.
1/4 in.
0.64 cm.
1 cm.
3/8 in.
1/2 in.
1.27 cm.
2 cm.
13/16 in.
1 in.
2.54 cm.
3 cm.
1 3/16 in.
2 in.
5.08 cm.
4 cm.
1 9/16 in.
3 in.
7.62 cm.
5 cm.
1 15/16 in.
4 in.
10.2 cm.
6 cm.
2 3/8 in.
5 in.
12.7 cm.
7 cm.
2 3/4 in.
6 in.
15.2 cm.
8 cm.
3 1/8 in.
7 in.
17.8 cm.
9 cm.
3 1/2 in.
8 in.
20.3 cm.
10 cm.
3 15/16 in.
9 in.
22.9 cm.
12 cm.
4 3/4 in.
10 in.
25.4 cm.
15 cm.
5 7/8 in.
11 in.
27.9 cm.
20 cm.
7 7/8 in.
1 ft.
30.5 cm.
25 cm.
9 13/16 in.
2 ft.
61 cm.
30 cm.
11 3/4 in.
