Why the Inverse-Square Law Might be ‘Wrong’ for Your Photo Shoot

Do you think you are familiar with the inverse-square law What is the practice photography equivalent of this “law?” The inverse-square law, which is the ultimate way to understand the laws of light, is taught to us. Many condense all of the amazing knowledge about light concepts into the inverse square. This article will show you how the inverse square laws is flawed. Sometimes.

According to inverse-square law, light falloff is proportional to distance from source squared. This relationship is common in physics and can also be found in field strengths. The inverse square relationship model is a popular method for computing all kinds of properties. Some photographers even go so far as to measure the exact distances to determine the power they will use.

A woman posing in a fashion photo against a red background

In most cases, the relationship is true. The distance between two points causes the light intensity to decrease. This is where modifiers can be a problem. Each modifier has its own light depth. This depends on the size and material of the modifiers.

I’ll show you how photography doesn’t have to be about numbers. Instead, it should be about light as a creative tool and not a way to create perfect images.

Why do we modify light in the first place?

To create an aesthetic, we modify light. Light can be moulded, sculpted and shaped. If placed too far, a hard reflector will increase light power and give direction to the light. A diffused softbox can make the light soft and even. We can then add reflectors and scrims to the mix. Each one will have an effect on the final result.

A woman in a fashion photo with her face obscured by shadow

The light was molded and sculpted at the top, so it might be more difficult towards the top than the bottom. However, the power for the right-hand side could have been reduced by half. Scrims can be used to reduce the power in a specific area of a white-on–white setting, for example when photographing products.

Photographers modify light to achieve the look they desire, not because of physics issues. It also breaks down if you add a modifier to your light or let something affect it, such as bounce.

Light Bouncing

In ideal situations, the inverse square law would create dark shadows due to the dramatic light falling close to the source of light. However, this is not always possible. This is true sometimes, but it doesn’t always happen.

Photographers work in small studios that have white walls. This is why there is a lot of light bounce. It can be difficult to get a dramatic falloff in small studios with little contrast. Moving the light source closer will not help. You will need to use flags and eliminate unnecessary light from your image. However, the inverse-square law is not always true.

A photo shoot in which a man is photographing two women

The Light Source is Too Close

Inverse square law presumes that the light source must be a point with a small size. This is no longer a reliable method of determining light output, as most photographers now use large softboxes.

Your light source must be larger than a bare bulb. It will not follow the inverse square rule because the source is larger and cannot be considered as a point. This law no longer applies to most practical uses and subjects, especially with large modifiers measuring meters in diameter.

A photo shoot with lighting equipment in the foreground

Anders Hanoola, David Bicho, and David Bicho have proposed a Five Times Rule. This rule states that it is reasonable to approximate a light source as a source of light if the distance between the source and the subject is five times greater than its largest dimension. This means that even if your softbox measures 5′ in diameter, the distance required for inverse square law’s behavior is at least 25 feet.

Although a 5′ octa is a common tool for photography, it’s not practical to use 25′ away as it will produce a very hard and dim light. These large modifiers are often used closer to the subject. You need to remember that light does not fall off at all distances. It is important to listen to your intuition and place the light where you want it to fall off.

Do not be a mathematician who uses a light meter. Be a photographer. Ideal without a lightmeter.

Modifiers come with different light levels. A softbox will have a different falloff than a telezoom, but it will be the same. Again, this is due to the size and way it throws light.

To demonstrate this, I did a little experiment. I used a bare-bulb flash and a magnum reflector. A 3′ octa was also included. And a 65” umbrella. Here’s what I got:

An example of light falloff with a 3 foot octabox
3′ Octa
An example of light falloff with a bare flash
Bare Flash
An example of light falloff with a magnum reflector
Magnum Reflector
An example of light falloff with a 65-inch umbrella
65″ Umbrella

Reflections Direct

Perfect direct reflections will reflect 100% of any light that is reflected on them. If you take a piece or metal, the intensity of the light will be the same regardless of distance. This sounds absurd, right? How is it possible for light to reflect the same intensity from the sun as the sun? Isn’t the light intensity affected by distance?

But it does. The reason is because the light spreads further and farther. Because light travels faster, it must cover more area. Direct reflection does not have an effect on distance. Only the size of the reflection changes the intensity.

A side by side comparison of light hitting metal

Although the inverse square law cannot be broken, it can be given a new interpretation by direct reflections.

Closing Thoughts

It is true that the inverse square law applies. If you are inclined to use mathematics with it, the relationship holds as long as all variables are taken into account. It is rare that you can calculate exact intensities or variables on a set. It is better to trust your instincts than use big modifiers and work with direct reflections.