Physics

Physics

They say you can’t fight city hall: but even city hall can’t fight physics. I’ll tell you why this matters on a Sunday, particularly, later…

Yesterday’s blog was about lighting, and those with a bit of knowledge of the area will have noticed that I was using lights rather close to the subject, which is how people often do it. It’s just that it’s not the best way, because of physics.

Physics underpins so much of photography, and it shouldn’t come as a surprise that physics isn’t always on your side. It certainly isn’t when you put a light a couple of feet away from your subject, because of the Inverse Square Law.

The picture at the top illustrates what the Law means for lighting. The wall that’s close to the flash unit is burnt out: off to the right, it’s getting underexposed. If you are using light like this for a portrait of two people standing shoulder to shoulder, one will be much more exposed than the other. If the light is two feet from one face and four feet from the other, there’s going to be a difference of two stops’ exposure between them.

Pull the light back to fifteen feet away, and you’ve got a very small difference. I like big studios and powerful flash units because of this! The sketch below may make this clearer. I hope…



You don’t have to understand the physics, but you do need to accept it, and account for it. It’s why checking exposures with a meter, or perhaps studying the histogram view of a test shot matters so much. If you have the space available, use it, and move the lights back, unless you want to use the exposure difference creatively.

Why’s this relevant to a Sunday? Remember school homework? I have a strong memory of an essay that had to end ‘And with a sigh, he tossed the black box over the edge of the cliff.’ I think I’d spent most of Sunday afternoon not writing it. It may be my autistic tendencies, but the problem simply didn’t compute. I still have the same problem with some sorts of test, exercise or competition, but I have worked out strategies that would let me write the story now.

But for a lot of people, physics was the big block between Sunday and going back to school for a new week. I look forward to hearing the best excuse for not handing in homework in first thing on a Monday morning…

And should you have a handy flashgun (or desk lamp), you may want to do your own homework by setting up a subject with heavy light falloff.