Inverse Square Law - Weekend

Inverse Square Law

By Albert Pedrosa

I NEVER thought I’d be writing about this particular branch of science that covers the characteristics of light. Because photography is all about capturing light, we need to understand how it behaves. It’s a bit technical if you try to put numbers on it and start making a mathematical computation out of it.

The inverse square law is surprisingly very much applicable to photography especially if you’re using artificial lights or strobes. It might seem obvious on how light behaves since we experience it every day, but you’ll be surprised to know that some of your understanding of light is quite different from what actually happens.

The inverse square law is defined as the intensity of an effect such as illumination or gravitational force changes in inverse proportion to the square of the distance from the source. Stop, don’t panic! This is a general law in physics that applies to any electromagnetic wavelength and that includes light.

Since photography is about capturing light, then maybe we should understand how this law will affect our lighting. Say you are shooting with a strobe or continuous light and you find that the light is too much, you move the lightsource farther double the distance, should you expect half the intensity? If your answer is yes, then you’re wrong.

Did this experiment using a speedlight, my trusty old light meter and a steel tape. You will notice that between 10cm to 40cm, the jump between apertures are significantly wide but on the 50cm mark onwards, aperture gaps between reading are relatively close.

No worries, my answer was also yes when I was asked the same question before. One evident scenario is when you position your artificial light source on one side and you’re shooting two subject side by side, the opposite subject will be underexposed. However, if the same two subject will be lighted by the sun on one side, both will have the same exposure.

Is it because the sun is a natural light source? The answer is no. The explanation has something to do with distance. The closer the light source to the subject, the faster the light fallout happens. Light fallout is the change of intensity of the light as it travels. In the inverse square law, double the distance means ¼ (one-fourth) power will be left and not ½ (one-half).

What’s interesting is that the farther the light source from the subject, the slower the light fallout. This explains why the two subjects side by side will have the same exposure even if the sun is on one side because the sun is quite far from the subject. Should you have the same scenario with your strobe, just move the light source as far as you can.

If your intention is to darken the background, move your light source as close as possible to your subject so your background gets the effect of inverse square law. Photography is always a troubleshooting situation, and understanding each and every element that affects our shot needs to be learned.

Let me know your thoughts and which topics you want me to talk about. Keep on shooting, everyone!

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