How do you calculate manual flash power when changing flash-to-subject distance?
Asked 8/20/2010
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2 answers
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If you keep ISO and aperture the same, how do you work out the manual power setting after moving a flash closer or farther from the subject? For example, using a flash with a guide number of 174 at ISO 200, shooting at f/8 gives a full-power distance of 174 ÷ 8 = 21.75 ft. If the room only lets you place the flash 10 ft from the subject, what power fraction should you set? Is flash power proportional to distance, or does it follow a different rule?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
16y ago
2 Answers
4
Its not linear, its a square, so if you double the distance, you need 4 times the light. So in your example you would need 1/4 the power, or adjust the aperture and or ISO.
Originally by user105. Source · Licensed CC BY-SA 4.0
user105
16y ago
0
Generated from our catalog & community — verify before relying on it.
Flash exposure does not change linearly with distance. It follows the inverse-square law: illumination is proportional to 1 / distance².
So if full power gives correct exposure at 21.75 ft and you move the flash to 10 ft while keeping ISO and aperture the same, you need less power by the square of the distance ratio:
power fraction = (new distance / original distance)² = (10 / 21.75)² ≈ 0.21
That’s about 21% of full power, which is closest to 1/4 power on a typical flash.
A useful way to think about it:
- half the distance = 1/4 the power
- double the distance = 4× the power
In practice, many photographers also fine-tune by taking a test shot and checking the result, then adjusting flash power as needed.
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AI16y ago
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