1

Theoretically, a combination of a speedlight of GN 45m@ISO100, an F1.1 lens, and a full frame camera capable of results at 51200 ISO should achieve a reach of almost a kilometer - or two when using 4 charges, four km with 16 charges to a bulb exposure.

Which factors will or can sabotage this naive calculation, apart from the need to keep stray light from the powerful flash from finding a direct way to the lens and causing flares, and obviously the need to take care not to cause blinding/startling hazards to bystanders and/or nearby traffic?

4

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.

1

Well, an F1.1 lens is not going to have a huge focal length, so you'll need a pretty large subject to get something useful on your camera at that distance. You'll want very clear air in order to actually get that reach, and very clear air tends to come with non-trivial amounts of star light and other illumination sources.

Then you likely want a nature scene really far from human dwellings because any kind of sign or cloth with a reflective layer will throw the received light back in a very focused manner, effectively halving the guide number calculation. Cats' and some other animals' eyes are also distractions. Signs and other "safety" reflective features are bad news particularly for direct flash.

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