﻿ T. Payne Photography | Inverse Square Law Put to Use

# Inverse Square Law Put to Use

October 18, 2014  •  4 Comments

Inverse Square Law - putting it together in the photography world.

The inverse square law tells us that the intensity of light is inversely proportional to the square of the distance from the source.

But what does that mean in simple terms that I can understand?

It means, in short, that the farther your subject is from the light, the less light that will be hitting your subject. Common sense right? See, you knew some physics and didn't even realize it. In photography, this is called light fall off. Now, to put it in practical terms and something that may be of use for us as photographers, this means that doubling the distance, reduces the light by approximately 1/4. (for you physics majors and math people, yes the decimals are slightly different, but for real world usage and keeping things relatively easy to understand, we will go with this approximation)

Our first project will be to take a photo of a ball. - settings: iso 100, shutter speed 1/125, aperture f/5.6 with your speedlight set at 1/4 power.

For the first part of the project, place the ball 12 inches from your speedlight at a 45 degree angle. Now, take a photo.

The results? Blown way out, right? That is because the intensity of the light at the close distance is way more than what we need to properly expose the ball. So, according to the inverse square law, we will need to move the speedlight further away from the ball in order to lower the light intensity.

Assuming that the inverse square law is accurate, if we move our speedlight back double the distance while keeping everything else exactly the same we can surmise that the light will be reduced by 1/4 the power that it was originally.

So, now it is time for the second part of our project.  Same settings, but now, move the speedlight back double the distance. So, we will go from 12 to 24 inches from the speedlight. Now that your speedlight is  24 inches from the ball. Take the photo..

What happened? Blown out again right? Did you notice a difference in how bad the photo was blown out? I know in my experiment, I certainly did.  We lowered our light intensity by 1/4 simply by moving the light back from the subject double the distance. In this case we went from 12 inches to 24 inches which caused a noticeable difference in the light intensity.

Now, it is time for the 3rd portion of our experiment. Remember the inverse square law? It says that by doubling the distance you lower the light intensity by 1/4 right? So, we were last at a distance of 24 inches from the ball. Now, we need to double that distance which will give us a distance from the ball of 48 inches. After backing the speedlight up to 48 inches, do a photo with all of the settings the same as our first : iso 100, shutter speed 1/125, aperture f/5.6, speedlight on 1/4 power.

Results? The photo looks a lot better than our very first one right? It is still overexposed a bit though. Do you see a dramatic difference between the 3 photos so far? Each time we move the speedlight back, we get closer to our desired level of exposure.

So, now we need to move on to the fourth portion of our experiment.. What to do? Keep all of the settings the same: iso 100, shutter speed 1/125, aperture f/5.6, speedlight on 1/4 power. This time move the light back double the distance again. So, we go from 48 inches to 96 inches. Take the photo and look at your results.

What does the photo look like? On my camera, this was a perfectly exposed photo. There were no highlights that were blown out and nothing over-exposed. Based on the light output of your speedlight, you should be somewhere close as well.

This shows how you can use distance from the lighting source to properly expose a photo. By moving our light source back double the distance, we were able to lower the light intensity by another 1/4 from before and we were able to get the exposure that we wanted.

By learning this technique, I hope you can now understand a little better as to the why and how light intensity affects your photos.

This is just our first project/experiment in learning about lighting. More  learning is right around the corner... subscribe to my blog to follow along..

Robin(non-registered)
This definitely had helped me to understand a bit more. Thanks
Stefanie Keeland(non-registered)
Thanks Tim!
Julie(non-registered)
Thanks Tim! Definitely going to follow all of your tasks to cement the basics in my mind!!
BILL KLAUS(non-registered)
LEARNED ALOT FROM JUST THAT, THANKS...