What is the Inverse Square Law? In photography it really just means the change in the intensity of light as a subject moves closer to or further away from its source.
In this quick guide we explain what the inverse square law means for photographers working with flash, and provide a handy table to help you understand the fall-off in illumination.
The reach of your flash is governed by a law of physics called the inverse square law. As light travels from your flashgun it doesn’t travel in a straight line, but spreads out in a cone shape to cover a wider area but with less intensity.
The inverse square law essentially says that for every unit of distance an object is away from your flashgun, the intensity of light it receives is 1/distance2.
So if we regard an object a metre away as receiving full power (1/1² = 1), then an object 2 metres away receives 1/2²=1/4 power.
At three metres it’s 1/3²=1/9 power, and four metres is 1/4²=1/16 power and so on.
The upshot is that light output falls off rapidly, so an object very close the flash will be lit much more brightly lit than one just a short distance behind it (as the table below indicates, an object two metres away from the flash only receives 25% of the light of an object one metre away).
However, the further the distance from your subjects to the flash the less critical this distance is (so place the same objects the same one metre apart from each other.
But five and six metres away from the flash, and they receive virtually the same amount of light – just 1% difference).
So if you’re photographing a group of subjects, don’t get too close.
But don’t get too far away either: once you get beyond ten metres or so then the flash is so weak that its effect is negligible, no matter how powerful the flashgun.
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