The Inverse-Square Law
In technical terms, an inverse-square law is defined as “any physical law stating that some physical quantity or strength is inversely proportional to the square of the distance from the source of that physical quantity.” With a definition like that, you’re probably wondering what on earth this could possibly have to do with photography (and no one could blame you). Inverse-square laws apply to many, many things in the world. Today however, we’re only going to be looking at one of them: light.
For those of us without an intense knowledge of advanced mathematics (or even very basic mathematics for that matter) something such as the inverse-square law can seem incredibly daunting.
The law itself, in photography, applies to lighting. It applies to any sort of lighting really but its most relevant application is with off camera lighting. In a nutshell, the inverse-square law teaches us how light works over distance and why the distance between your light source and your subject is so important.
Let’s say we have a light source which is on full power and our subject is 1 meter away it. If we move our subject double the distance away from the light (2 meters), how much of the light’s power will reach it? The natural reaction is to think “half power” – but unfortunately that’s now how light works, it follows an inverse-square law.
According to the law, the power of the light will be inversely proportional to the square of the distance. So if we take a distance of 2 and square it, we get 4, the inverse of which would be 1/4 or rather, a quarter of the original power – not half.
Moving our subject 3 meters from the light (3 * 3 = 9, so 1/9) the power of our light source now becomes 1/9th of what it originally was.
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