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An ƒ-number indicates the relative aperture of the lens and lets us
determine how much light it lets through. But why does the number get
bigger when the amount of light decreases? The answer is in the way
it is written, ƒ/2.0, ƒ/8.0. etc. That means the focal length of the lens (ƒ)
divide by 2 or 8. So a 100mm lens at ƒ/2 has a 50mm physical aperture
(100/2) and at ƒ/8, a 12.5mm physical aperture (100/8). So the f-
number indicates the size of the aperture as a fraction of the focal
length of the lens.
The ƒ-number series was carefully chosen to make controlling
exposures easy. In the chart, the aperture and shutter speed
combinations give the exact same exposure. Notice how the shutter
speed changes to compensate for the change in aperture.
55
CAMERA SENSITIVITY AND FLASH RANGE
ƒ/2.8ISO setting
100
400 / AUTO
800
1600
1.0 ~ 4.3m / 3.3 ~ 14ft.
1.0 ~ 6.0m / 3.3 ~ 20ft.
2.0 ~ 12m / 6.6 ~ 39ft.
2.8 ~ 17m / 9.2 ~ 56ft.
ƒ/5.6ƒ/4.0
3200
4.0 ~ 24m / 13 ~ 79ft.
1.0 ~ 3.0m / 3.3 ~ 9.8ft.
1.0 ~ 4.3m / 3.3 ~ 14ft.
1.0 ~ 6.0m / 3.3 ~ 20ft.
1.4 ~ 8.6m / 4.6 ~ 28ft.
2.0 ~ 12m / 6.6 ~ 39ft.
2.8 ~ 17m / 9.2 ~ 56ft.
1.0 ~ 2.1m / 3.3 ~ 6.7ft.
1.0 ~ 3.0m / 3.3 ~ 9.8ft.
1.0 ~ 4.3m / 3.3 ~ 14ft.
1.0 ~ 6.0m / 3.3 ~ 20ft.
1.4 ~ 8.6m / 4.6 ~ 28ft.
2.0 ~ 12m / 6.6 ~ 39ft.
A change in camera sensitivity (ISO) affects the flash range. The flash range depends on
the aperture used:
Low80
1.0 ~ 3.8m / 3.3 ~ 12ft.
1.0 ~ 2.7m / 3.3 ~ 8.9ft. 1.0 ~ 1.9m / 3.3 ~ 6.2ft.
200 / Hi200
1.4 ~ 8.6m / 4.6 ~ 28ft.
What is an ƒ-number?
f/2.8 1/2000s
f/4.0 1/1000s
f/5.6 1/500s
f/8.0 1/250s
f/11 1/125s
f/16 1/60s
f/22 1/30s
f/32 1/15s