Chances are that you may not have heard about the inverse square law. But you must have seen the effects of the inverse square law in photography. The inverse square law deals with the light falloff in photography. It has got further ramifications in other branches of science, but for the sake of photography, we will only deal with its effect on light falloff.
What is light falloff?
Light falloff is a term used in photography to demonstrate the dropping of the intensity of light as you move further away from the light source. That means the intensity of light when you are close to the light source will be higher than when you are, say, 10 feet away from the light source. A mathematical calculation calculates how much light loss happens when you move away from the light source.
Inverse square law states that light intensity is equal to 1 over the square of the distance. Let’s say a subject is lit by a light source positioned one foot from the light source. Now when we move from one foot to two feet, the intensity of light drops. By how much? It can be calculated by using the mathematical expression I shared above.
Inverse square law
intensity of light = 1 / square of the distance
intensity of light = 1/2² or ¼ or a quarter of the light you received at one foot.
In this case, as we double the distance from the light source, the light intensity drops by 75%.
The exciting thing about this mathematical expression is that it is irrelevant what measuring unit you use. Whether it is feet, meters, or centimeters, as you double the distance, the light drops by 75%, which is quite a bit in photography.
Now the million dollar question is what is the utility of this mathematical expression and how it impacts our photography? When we know the amount of light follows that happens because of increasing the distance from the subject to the light source, we can use that to calculate the correct exposure. Now, as you have seen above, moving from one foot to two feet, the light falloff is the most intense. as you go further away from the light source, the intensity of the light falloff is not that much.
This has an interesting application. Let’s say you want to capture a high-contrast, high-intensity portrait image. You want to capture a lot of shadows going with many highlights in the image. To capture such a photograph, you must position the light very close to where the subject is.
On the other hand, if you wish to create a flattering portrait where the subject is well-lit, position the light further away from the light source.