Inverse Square Law

Inverse Square Law

The fifth variable of flash exposure is called the Inverse Square Law.

It just sounds scary, but it's not. We can go into an extremely complicated explanation that the physicists of the world would be proud of, but all that would do is confuse you even more. In our artistic photographers eyes, the simpler the better.

The Inverse Square Law when applied to photography relies on two principles. That light diminishes over distance and that the further away your light source from your subject the larger your area of correct exposure is going to be. In scientific terms it states that every time the distance of the light is doubled, you have 1/4 of the intensity of light. In photographic terms it means that every time the distance the light travels is doubled, you lose two stops of light! It sounds more complicated than it is, so just relax!

The first principle of the inverse square law is that light diminishes over distance. The power of the light is not actually diminishing, it's dispersing. This means that the power is the same, the light is just more scattered everywhere making it less intense. Think of it like a flash light. The further the throw of the light is, the less the flashlight lights up but the wider the beam becomes. This is because light is dispersing more and more the further the light travels. The dispersing of the light is what makes it lose intensity. The reason a laser is so powerful at such long distances is because it prevents that dispersion and the beam is concentrated for longer distances.

An important thing to note is that the further the light source is from your subject, the harsher the light is going to be, the closer the light source to your subject, the softer the light. I will go over this in more detail in another article, just keep that in the back of your mind for now!

The main point to remember is that the further that light travels, the less power it's going to have when it hits your subject!

The second point, which is very important, is that because of the dispersion light will become more even over distance and fall off less quickly. What is fall off? When photographers are talking about light fall off, they are referring to the evenness of the exposure. When light has a fast fall off, it means that your area of correct exposure is very small. That your light is falling off of your correct exposure area very fast. In the diagram above you can see that I broke it down into different segments, each segment shows the Aperture you need to have a correct exposure in each area. Notice that the further away the light travels the larger the correct exposure area becomes but the less intense the light is. Every time the distance the light travels is doubled, you lose 2 stops of light!

This is what the inverse square law is all about. The closer your light source is to a subject, the faster the light is going to lose intensity causing a faster fall off. The further the light source is away from your subject, the slower the light is going to fall off, which increases your correct exposure area. For example, look at the illustration above. If I'm shooting a group of people and I put my light source very close to them, because of the size of the group they fall into two different exposure sections. On my camera I can't expose for two different sections at one time so I have to choose. If I choose F/16 as my correct exposure, the person in the F/8 section is going to be 2 stops underexposed. If I choose my exposure to be F/8, then the person in my F/16 section is going to be 2 stops overexposed.

The key is to move your subjects far enough away from the light source so that they fall into the same correct exposure area like in the illustration above.

The point behind all of this, and you don't have to bring out a tape measure, is that the further away your light source is from your subject, the larger area of correct exposure you are going to have, the closer, the smaller.

Think of it like a flashlight lighting a car. If you put the flashlight really close to the car you will have a bright light lighting up a small area of the car, but if you step back far enough to light up the entire car, you will light it all evenly, but it will not be as bright because light diminishes over distance!

Creatively this is extremely useful to know because you can cause a dramatic fall off on your subject by moving the light closer to them, or a more even light by moving it away! You can also control the light that is hitting your background by moving the light source closer or farther away. For example, if I wanted to make the wall behind my subject as dark as possible, I would move the subject and light source as far away from the wall as I could. In the illustration above my camera would be set to F/16 because that is my subjects correct exposure. The wall is at an F/2 correct exposure. That means that since my camera is set to F/16 the wall whose correct exposure is F/2 is going to be 4 stops underexposed.

If I wanted to make the background as evenly lit as possible, I would have to move my light-source away and put my subject and wall in the same correct exposure section like the illustration above. The important thing to remember and pretty much all I use when in the field is that the closer the light-source, the smaller my correct exposure area is going to be but the brighter the light, the further the light source is, the larger my correct exposure area is but the less bright the light is going to be!

There is just one more important thing you should know about the inverse square law and it's the Size of your Light Source Relative to your Subject. This is important for one extremely important thing: the quality of the light. Light can either be soft or hard. When we talk about light being soft or hard we aren't actually talking about the light, but the shadows. When shadows are very defined like they are in the midday sun, we consider this hard light. When the shadows have a very feathered edge or non existent, we consider this soft light. Even though the light really doesn't change because a photon is a photon, the shadow is what makes it seem hard or soft. When we put a strobe inside of a softbox what we are actually doing is increasing the size of the light-source to make it larger than the subject you are illuminating. The larger the light-source the further back you can put it while still maintaing a soft light.

The softness or hardness of light is going to depend on the the Size of your Light Source Relative to your Subject. The larger the light source is in relation to your subject, the softer the light is going to be. This is because the larger the light source, the more chance the light has to wraparound your subject and create feathered shadows. The smaller the light source is in relation to your subject the harder the light is going to be because the light does not have the ability to wrap around your subject and feather the shadows.

Also the closer you put your light modifier to your subject, the softer the light is going to be. The farther away you put your light source from your subject, the harder the light is going to be. This is because of perspective. Think of it like the sun, when it's bright and sunny with no clouds in the sky the light is very hard. The sun is a huge light source, but since it's so far away it's very small relative to you which causes the very hard shadows. When it's an overcast day, the sun lights the clouds and the clouds become the light source. The clouds are huge relative to you so the light is very soft, because the size of the light source (the clouds) is much bigger than you are, the shadows are feathered, or may be non existent.

A modifier works the exact same way, the larger it is in relationship to your subject, the softer the light. So in reality it's not really how far the light modifier is, it's how big the light source is in relation to your subject. You can have a large 9 foot parabolic umbrella, but if you put it so far back that it's the size of a 32 inch umbrella, the light quality of the two is the same. If you are using a speedlight and are photographing an ant, that speedlight is huge compared to the ant, so the light you are illuminating the ant with is extremely soft.

So moral of the story, when it comes to light, SIZE MATTERS!