The Inverse Square Law: creating clean portraits with physics

Articles about light and how to use it in photography are often anything but inspiring. They are filled with technical diagrams, charts, and equations. And if you are into science and math they are super helpful!

But that’s not me (I may or may not have almost failed high school calculus!)…and that’s not this article. Yet I believe that even a less-than-enthusiastic math student like me can understand complex light concepts and why they work. Even better, I believe we can be inspired by what light can do in our photographs and create new, exciting work as a result!

The Inverse Square Law (ISL) sounds pretty scary on the surface. Anything with theorem or law or equation in the title usually makes me want to run for the hills! But I promise to keep the science lesson short because ISL is awesome. This magic little concepts will allow you create a clean scene in even the most cluttered of homes!

A little science and math

The Inverse Square Law states “the intensity of light is inversely proportional to the square of the distance from the light source.” For the maths lovers, that formula looks like this: Intensity = 1/Distance2

I can feel you rolling your eyes. I am right there with you!

Don’t leave me just yet! That crazy-sounding concept is actually pretty simple. All that means is that the further your subject is from the light source, the more gently light will fall on it.

It also tells us that the intensity of the light diminishes exponentially, so thing farther away get darker sooner than you might expect. This is because as light travels across distance it spreads out. So less and less of the original concentrated light power hits the subject.

So, a subject that is twice as far from the light source as another does not receive half the amount of light as logic would tell us it should. Physics is in no way intuitive – gee thanks science! Instead, it receives a quarter of the amount of light (thus the ‘square’ in Inverse Square Law).

OK – I promise to stop with the science explanations here. Let’s get into how we can USE these concepts!

Seeing how it works

What you need to remember is that the closer your subject is to the light source, the deeper the shadows will be and the closer they will appear to your subject. This dramatic interplay of light and shadow is called Chiaroscuro (I may not be a science fan but I do love art history!). Let me show you what I mean:

Here we see the change in light and shadow as the light source moves further away from the subject one foot at a time. Notice how in each sequential image the light disperses, becoming softer. As a result the shadows become less dramatic and the light illuminates more of the hair and ears.

Putting physics into practice

My favorite application of the ISL is to allow it to act as my personal maid. It allows me to erase the clutter of my lived-in home (littered with Legos, and crayons and cars) and make it disappear into lush and velvety nothingness.

All I have to do is put my subject closer to the light!

This principle is often applied in-studio to make a white backdrop appear grey or even black. This is accomplished by keeping the subject close to the light source while increasing their distance from the backdrop. As the subject inches away from the backdrop and toward the light, a white background can move along a gradient of gray all the way to black.

The same idea transforms an otherwise busy background into a ‘fake black’ background!

I captured the following images seconds apart from each other. The camera distance and natural light source remained the same throughout. All that changed was my distance (as the subject) from the light source. The result is a significant change in the darkness of the background area.

Can you believe those were taken here?!

When I am close to the window the background appears very dark and my face looks brightly lit and contrasty. (Gotta love those harsh light wrinkles!) As I move further from the window the light disperses more evenly but there is still some distinction. When I move back into the depths of the room there is less differentiation between the light hitting me and the light hitting the background scene. As a result you can see all the clutter.

To employ the Inverse Square Law and allow it to erase the clutter, you want your background far from the light source and your subject near to the light source. You also want to keep other light sources from leaking into the background space.

A step-by-step guide

Your garage or hallway is a great place to practice this technique. This is my step-by-step process:

1.) Look for an area that has a bright light source (a window or doorway) and no light leaking into the background area. You may need to cover windows in the rear of the space.

2.) Move your subject very close to the light source and as far as possible from the background.

3.) If you are trying this in your garage with the door open look for the line on the floor where light and shadow meet. You want to place your subject just inside that line of shade (not on the sunny side).

4.) The messy background will receive only a tiny fraction of the light that your subject is getting. So when you expose for your subject, all that background mess dissolves into rich shadow.

5.) Spot meter to expose for the brightest part of the subject’s skin.

6.) Take your photo and marvel at your greatness.

7.) Dodge, burn, and clone as necessary in the editing program of your choice.

This example was in our filthy garage. Our garage has windows at the rear so I had to cover those up. I also had to do a little extra burning in post to make that background perfectly dark. But would you believe that our tumble dryer is actually back there in that darkness?

I took this portrait in our lounge next to glass double doors. So that no light could enter the back of the room, I closed the curtains.

I shot this example in my kitchen which has dark cabinetry and white tiles and benches. I brought my son very close to the window and exposed for the highlights in the slime. This meant I was able to lose the background in darkness. His skin was quite underexposed as a result, so I brought the highlights and skin tones back up in post processing.

So that’s it! We’ve played with physics and there’s not a pocket protector in sight. Even better, all that math and science makes it so that we can live in our homes AND create beautiful, clean portraits. Physics for the win! So go out, create, and with the Inverse Square Law know that you now get a perfect score in being fantastic!