Think you know the inverse-square law? How much of a “law” is it actually in the practice of photography? We are taught that the inverse-square law is the holy grail of understanding the laws of light. Some condense all the wonderful knowledge about light concepts to the inverse square law only. In this article, I want to invite you to take a step back and see how the inverse square law is wrong. Sometimes.
Inverse-square law says that light falloff is inversely proportional to the distance from the source squared. This is a common physics relationship that is also found in things such as field strengths. Physicists take great pride in being able to calculate all sorts of properties using the inverse square relationship model. Some photographers go as far as to measure exact distances and try to figure out the power they will use.
The relationship itself is true in most applications. Light intensity does decrease as distance increases. The problem arises when photographers use modifiers. Each modifier has a different light depth. That depends on their size, material, as well as other things.
I will try to show you that in photography you really don’t need to think about numbers — instead, you need to think of light as a creative tool rather than a mathematical way to get perfect images.
Why Do We Modify Light in the First Place?
We modify light in order to create an aesthetic. Light is a free form that can be molded, sculpted, and shaped. A hard reflector will give light direction, increase power, and make it hard if placed far enough. A large diffused softbox will make the light soft, even, and less directed. Then we can introduce things such as reflectors, scrims, and diffusers. Each of them will have an impact on the final result.
So by the point the light had been molded and sculpted, it may be harder towards the top of the frame than the bottom, the power could’ve been halved for the right-hand side too. For example, if you’re photographing products, you may want to use scrims to decrease power on a particular part of a white-on-white setting.
The reason photographers modify light is so that they can achieve the look they want, not because they want to test the physics relationship. Moreover, it breaks down the moment you put a modifier on your light or have something influence the light, for example, bounce.
In an ideal situation, the inverse square law will create dark shadows because of the dramatic light falloff close to the light source, and more even falloff at greater distances. That is true, but sometimes this doesn’t happen.
Most photographers work in small studios with white walls, hence there is expected light bounce. Getting a perfect dramatic falloff with lots of contrast in a small studio is rather hard, and moving the light source closer won’t exactly do the trick. Thus, you need to work with flags and remove unnecessary light from your image. The inverse-square law holds, but it is altered in an unexpected way.
The Light Source Being Too Close
The inverse square law assumes that the light source is a point that has a negligible size. However, with most photographers opting to use big softboxes, this can no longer be a reliable way of telling light output.
If your light source is anything bigger than a bare bulb, it won’t follow the inverse square rule as the light source is bigger and cannot be considered a point. With huge modifiers that are meters in diameter, this law is no longer true for most practical purposes and subjects.
Nonetheless, there is a certain Five Times Rule, proposed by Anders Hanoola and David Bicho. It states that it is fair to approximate light source as a point source if the subject to light distance is five times greater than the light’s largest dimension. What this means is that if your softbox is 5’, you need to be at least 25’ away for inverse square law to behave how it would with a bare-bulb source.
A 5’ octa is very common to use in photography, but 25’ away it becomes a lot less practical, as it will give a fairly hard and dim light. Large modifiers such as this one are commonly used closer to the subject. When working with such cases, you need to be aware that although light falls off with distance, it won’t be perfect. Hence, following your gut and placing the light in a position where the falloff is what you’re looking for is key.
Don’t be a mathematician with a light meter. Be a photographer. Ideally without a light meter.
There’s a reason modifiers have different light depths. The falloff is different from a softbox than it is from a telezoom reflector. This is again due to the size of the light source and how it throws light.
I’ve done a little experiment to show you this. I took a bare-bulb flash, a magnum reflector, a 3’ octa, and a 65″ umbrella. Here is what I got:
A perfect direct reflection will reflect 100% of the light that is cast on it. So, if you take a piece of metal, it will reflect the same intensity as the light source, no matter the distance. That sounds a bit stupid right? How can something reflect light with the same intensity as it leaves the sun? Doesn’t light intensity decrease with distance?
It does, but the reason it does is that the light spread is larger and larger. So, as light travels, it has to cover a larger and larger area. However, with direct reflection, the intensity does not change with distance, only the reflection size does.
The inverse square law is not broken, but it is given a different interpretation with direct reflections.
The inverse square law is certainly true, and if you are inclined to be mathematical with it, the relationship stands as long as you take all necessary variables into consideration. However, it is very rare to be calculating exact intensities and variables on set. Following your gut is a much better way to think of the inverse square law, especially when using big modifiers or when working with direct reflections.