Chapter 3 ✦Flash Photography Basics 71

Aperture
Another factor that determines the proper
flash exposure is the aperture setting. The
wider the aperture, the more light falls on
the sensor. The aperture or f-stop number is
a ratio showing the fractional equivalent of
the opening of the lens compared to the
focal length. Are you confused? It’s actually
pretty simple:
The lens opening at f/8 is the same as 1/8
of the distance of the focal length of the
lens. So, if you have a 50mm lens, the lens
opening at f/8 is 6.25mm. 50 divided by 8
equals 6.25.
All of the math aside, all you really need to
know is this: if your Speedlight output is
going to remain the same, in order to lessen
the exposure, you need to stop down the
lens to a smaller aperture or move the
Speedlight further away from the subject.
Distance
The third part in the equation is the distance
from the light source to the subject. The
closer the light is to your subject the more
exposure you have. Conversely, the further
away the light source is, the less illumina-
tion your subject receives. This is due to the
Inverse Square Law, which states that the
quantity or strength of the light landing on
your subject is inversely proportional to the
square of the distance from the subject to
the Speedlight.

Table 3.1

SB-600 Guide Numbers (at ISO 100)

Flash
Output
Level Zoom Head Position
14*242835 507085
M 1/1 14.0 M 26.0 M 28.0 M 30.0 M 36.0 M 38.0 M 40.0 M
45.9 Ft 85.3 Ft 91.9 Ft 98.4 Ft 118.1 Ft 124.7 Ft 131.2 Ft
M 1/2 9.9 M 18.4 M 19.8 M 21.2 M 25.5 M 26.9 M 28.3 M
32.5 Ft 60.4 Ft 65.0 Ft 69.6 Ft 83.7 Ft 88.3 Ft 92.8 Ft
M 1/4 7.0 M 13.0 M 14.0 M 15.0 M 18.0 M 19.0 M 20.0 M
23.0 Ft 24.7 Ft 45.9 Ft 49.2 Ft 59.1 Ft 62.3 Ft 65.6 Ft
M 1/8 4.9 M 9.2 M 9.9 M 10.6 M 12.7 M 13.4 M 14.1 M
16.1 Ft 30.2 Ft 32.5 Ft 34.8 Ft 14.7 Ft 44.0 Ft 46.3 Ft
M 1/16 3.5 M 6.5 M 7.0 M 7.5 M 9.0 M 9.5 M 10.0 M
11.5 Ft 21.3 Ft 23.0 Ft 24.6 Ft 29.5 Ft 31.2 Ft 32.8 Ft
M 1/35 2.5 M 4.6 M 4.9 M 5.3 M 6.4 M 6.7 M 7.1 M
8.2 Ft 15.1 Ft 16.1 Ft 17.4 Ft 21.0 Ft 22.0 Ft 23.3 Ft
M 1/64 1.8 M 3.3 M 3.5 M 3.8 M 4.5 M 4.8 M 5.0 M
5.9 Ft 10.8 Ft 11.5 Ft 12.5 Ft 14.8 Ft 15.7 Ft 16.4 Ft
* With wide flash adapter
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