Does flash guide number still follow the inverse-square law at very long distances?
Asked 11/6/2019
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If you take a speedlight rated around GN 45m at ISO 100 and combine it with a very fast lens and very high ISO, does the usual guide-number math still apply at unusually long distances, or do other factors make it break down? Aside from flare from direct spill into the lens and obvious safety concerns, what practical limits would sabotage this calculation in real-world shooting over very long range?
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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I guess there would be some some small amount due to absorption in the atmosphere. But in theory there's not much complicated to it, and I don't see why it wouldn't still be the same. The formula works because light follows the inverse square law — and that works for studio lights in a small space just the same as it does for stars. (I mean, the miasmas of incandescent plasma, not celebrities.)
However, the caveat — and it's a huge one — is that in the real world at that distance, I don't think your flash output on the subject is going to be a significant factor compared to all the ambient light.
After all, the amount of light your GN 45m flash is providing at that distance is about the same as GN 0.14m flash would throw across a 3m room. For comparison, a typical camera's built-in pop-up flash has a GN of 14m or so — something like 10,000× that power. You can work out the math further if you like; I think that's enough for the intuitive point.
So, if you can find a situation where you have pitch black darkness yet 4 km of clear air, go ahead with this plan. Otherwise, don't even bother with the flash.
Originally by user1943. Source · Licensed CC BY-SA 4.0
user1943
6y ago
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Generated from our catalog & community — verify before relying on it.
In principle, yes: guide-number calculations are still based on the inverse-square law, and that physics does not stop working just because the distance gets very large.
What changes in practice is everything around the calculation:
- Ambient light becomes a major problem. At extreme distances, the flash contribution on the subject may be so weak that moonlight, starlight, city glow, or other illumination matters more.
- Atmospheric clarity matters. Haze, dust, moisture, and general air quality reduce contrast and effective reach.
- Subject size and framing matter. A very fast lens like f/1.1 usually is not a long telephoto, so at great distance the subject may be too small to photograph usefully.
- Highly reflective materials can return light disproportionately, giving misleadingly bright spots even when most of the scene remains dark.
- Very high ISO and long exposures may record ambient light and noise more than useful flash illumination.
So the GN rule is still valid as a starting point, but at very long distances the limiting factors are usually ambient light, atmosphere, subject size, and reflectivity—not a failure of the guide-number formula itself.
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