The Inverse Square Law Cheat Sheet – Myth Busted

The Inverse Square Law Cheat Sheet UPDATE: This Experiment is all Wrong. I should hit my head on the same wall I used to measure reflected light off. Some great comments about what went wrong, and great discussion going on – I posted the main points here.

Have you heard about the Inverse Square Law? It’s the law that says that light intensity falls the farther you move your light from your subject. It also tells you that if you move your light to be twice as far it will fall by 4 (the square of 2). if you move the light three times as far, it will fall by 9.

We all swear by that law. The only thing is this law does not apply to the way most of us use flashes.

I’m gonna explain this in a beat, but first here is my newest cheat sheet. (I love cheat sheets. If you are as senile as me, you can print them and then pop them up later and look really smart).

The Inverse Square Law Photography Cheat Sheet

You can see it really, really, really big by clicking here.

How’s This Card Working & Some Myth Busting

The right lobe of my brain always told me that there was something a bit off with the way we apply the inverse square law to strobed shooting. I mean the Inverse Square Law applies to light that expands in a sphere like shape. Usually our strobes are not spreading a sphere of light, but a beam of light. (Ok, is too long – it’s going to be ISL from now on).

So I set out to test this. This card is made from four different scenarios – each show how light expands (and falls off) depending on the way we set our strobe.

The markers on the top represent the distance from the light source AKA Strobe (each mark is 50 cm). The words on the right tell the scenario under test.

First – the good news. When we use a stofen we get what we paid for – ISL in action. With a Stofen, the light is spreading from the flash in a very spherical way – hence when we double the distance from the flash (from 1 meter to two meters) we get aproximetelly1/4 of the light intensity.

Now, the not so good news – Whenever we set out flash on zoom we can throw ISL out of the window. The focusing of the light burst makes the light behave more like a narrow beam and less like a sphere. When we double the distance from one meter to two meters we lose just a little bit of light. When we double the distance again – it makes more sense, yet sampling dots on the 105mm line deviates far from ISL.

Actually, this is not really bad news – it means that we can get some nice power far away if we focus the beam.

Now, I’ll admit, there’s some sampling error when we are close to the strobe – please take the big version and see for yourself.

The Making Of

Luckily, I am still leaving on a construction site, this makes it easy for me to find long stretches of empty walls. The nice thing about it is that you can see the blocks – each block = 50 centimeters.

I placed the strobe on a lightstand (the 001b)  and placed my D300 on a far away tripod. Strobe was triggered with poverty wizards (AKA cactus triggers). 

I took a set of images that look very much alike – the only diff was the flash settings / modifier mount.

For the card I used the middle strip of each image

The Inverse Square Law Cheat Sheet

The blocks made it easy to mark lines 50cm apart.

For light intensity I used Photoshop‘s sampling tool.

Sharing Is Caring 

As with my other Cheat Sheet Card, I placed the card on a CC license – feel free to print it, share it, use it in class and distribute it in general.

You may even use it to tell me I am wrong

More Reading: