Guide Numbers for different focal length ranges (at ISO 100, in meters / feet)
Flash Coverage (mm)
Flash
Output
14
24
28
35
50
70
80
105
1/1
15/49.2
28/91.9
30/98.4
39/127.9
42/137.8
50/164
53/173.9
58/190.3
1/2
10.6/34.8
19.8/65
21.2/69.6
27.6/90.7
29.7/97.4
35.4/116.1
37.5/123
41/134.5
1/4
7.5/24.6
14/45.9
15/49.2
19.5/64
21/68.9
25/82
26.5/86.9
29/95.1
1/8
5.3/17.4
9.9/32.5
10.6/34.8
13.7/45.2
14.8/48.6
17.7/58.1
18.7/61.4
20.5/67.3
1/16
3.8/12.5
7/23
7.5/24.6
9.7/32
10.5/34.4
12.5/41
13.3/43.6
14.5/47.6
1/32
2.7/8.9
4.9/16.1
5.3/17.4
6.9/22.7
7.4/24.3
8.8/28.9
9.4/30.8
10.3/33.8
1/64
1.9/6.2
3.5/11.5
3.8/12.5
4.9/16
5.3/17.4
6.3/20.7
6.6/21.7
7.3/24
1/128
1.3/4.3
2.5/8.2
2.7/8.9
3.5/11.4
3.7/12.1
4.4/14.4
4.7/15.4
5.1/16.7
