Why doesn’t shutter speed appear in the manual flash guide number formula?
Asked 11/5/2019
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2 answers
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When using a manual flash, exposure is often calculated with the guide number formula:
guide number = f-number × distance
for a given ISO.
Why isn’t shutter speed included? Intuitively, a longer shutter time should make the image brighter and a shorter time should make it darker. How does shutter speed affect exposure when using manual flash?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
6y ago
2 Answers
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Flash duration is typically much shorter than most cameras' flash sync speed.
If the flash only has a duration of, say, 1/1000 second (or 1 millisecond), it matters not if the shutter is open 1/250 second (4 milliseconds) or 1/25 second (40 milliseconds), the energy from the flash that is captured by the photo will be the same in either case.
What shutter duration does affect is the amount of ambient light recorded. But your formula is strictly for calculating the light energy from a flash and does not include anything about ambient light or the ratio between ambient light and flash.
Originally by user15871. Source · Licensed CC BY-SA 4.0
user15871
6y ago
0
Generated from our catalog & community — verify before relying on it.
Shutter speed isn’t in the guide number formula because that formula describes flash exposure, not ambient-light exposure.
A flash burst is usually very brief—often much shorter than normal flash sync speeds. If the flash fires while the shutter is fully open, the sensor receives essentially the same flash energy whether the shutter is open for 1/250 s or 1/25 s. So flash brightness is mainly controlled by:
- flash power
- subject distance
- aperture
- ISO
That’s why guide number calculations use aperture and distance for a given ISO, but not shutter speed.
What shutter speed does change is the amount of ambient light recorded. A longer shutter lets in more room light, daylight, or other non-flash light; a shorter shutter reduces it. In a very dark scene where the flash is the only meaningful light source, shutter speed has little or no effect on subject brightness from the flash.
The main requirement is that the shutter speed be at or below the camera’s flash sync speed so the whole sensor is uncovered when the flash fires.
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