Inverse Square Law for Photographers

inverse-square-law.jpg Light is like paint. Yep thats right, paint. Imagine you have one can of paint and you are going to use all of it. In this diagram, you can see that light spreads as it travels out. So if you were to paint any of those surfaces with the whole can of paint, it will be thicker on the smaller surfaces. Well, thats brightness. We are painting with Light!

I am not going to get into formulas, and there area few to determine light on subject vs background etc... but, nope, not going to do it here. I am simply going to show you how to understand whats going in relation to your photos.

To try and simplify, I tried to recreate the image of light spreading and put this on top of a studio. Feel free to click the image to make them bigger.

In all these shots, the light brightness stays the same, so you would have to adjust your exposure for the shot, but I want to show the difference in brightness depending on position:

model-distance-a.jpg

In this shot: Model is close the the light, and background is 2x the distance, the background would be about 4x darker than the model.

model-distance-b.jpg

Move background: Now, we move the background back even further, the model is the same brightness because the model did not move. Now you can see the background gets much darker than the model. Don't worry its not important (right now) that you understand the exact ratio, but rather understand whats going on.

model-distance-c.jpg

Move model close to background: Now we move the model closer to the background. But you should notice that the distance from the model to the background is the same as the first example. The only difference is both the model and background are moved back. Because of this, the amount of light on the model and background is very similar.

model-distance-d.jpg

Black Background: By moving the model a little closer to the light, and moving the background further back, we can blackout the background from the shot.

It is all about the distances, and many ways to describe it, but I keep it simple and look at it this way:

Distance from the light to model I will call A and Distance from model to background I will call B

  • If the distance from the light to model is smaller than the distance from the model to background, then the difference in brightness will be grow as the light moves closer
  • If the distance from the light to model is greater than the distance from the model to background, then the difference in brightness will get smaller as the light moves away

Best way to understand better, is to go some flash photography. If you are using constant lights, make sure room lights are low, and you only have the light from one source to work with.

Recreate the above shots and you will start to see the differences.

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Mike Bradley

Author: Mike Bradley

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