On the way into work this morning I was thinking about dark background flash photography. Essentially I was wondering how you would work out how far away your background would have to be from your flash and subject in order for the background to be recorded as completely black.
I’m sure you’re all well aware of the rule that light falloff follows an inverse square law. It also occured to me on this drive that f-stops on a lens also follow a square law. This led me to wonder the following:
If you know the dynamic range of your sensor in terms of stops – for example 9 – and you know the distance from you flash to your subject – 1m – is the black level distance really very easy to calculate? 4.5m beyond subject.
Let me walk you through what I’ve just done:
I’m assuming (incorrectly but this is just a quick example) that your correctly exposed subject falls dead in the middle of your dynamic range, i.e. 4.5 stops from peak white and peak black. I’m also assuming that if you double the distance between your subject and your flash (to 2m) that you’d have to open your aperture up one stop to compensate. This would give evidence to my idea that the square law fall of light is directly proportional to the square law nature of ‘stops’ in camera lingo. My final assumption is that if you took a meter reading and the underexposed by 4.5 stops (in this case with a dynamic range of 9) that you would get a pure black output.
All of this is just conjecture at the moment, but I have an idea of how to test it… more info when I get the time to experiment 🙂
If your flash is 16 feet away (no surprises on the number), then 8ft behind the subject is 1 stop down, 4ft after that is another stop, 2 ft after that is another stop and 1ft after that is another stop and that last foot the last stop. 4 stops, in 15ft of distance.
So you’d need to have the flash 32ft from the background to have the subject in the middle of the 9 stop range, and the flash be dead and cold by the time it hit the backdrop.
I think.
This doesn’t seem to tally well with our lounge experiments, though… so maybe I’m confused too… that is unless your face was actually higher in the DR range than that, so we needed to drop more, maybe 7 to 8 stops of light…
Wow, now I’m really confused. Don’t forget that the inverse square law falloff is only applicable to point sources of light – which we didn’t have. I may have to experiment a bit.