The inverse square law teaches us how light behaves over distance and why the distance between your subject and your light source is so critical when making photographs.
The power of the light is inversely proportional to the square of the distance. In other words, double the distance, lose two stops of light.
Using 3 18% gray cards, we can see that the light is strongest when it hits the front card, making it appear lighter than the back two cards. The card farthest away appears much darker than those in front of it. This is because the light falls off (appears less intense) as it moves across a distance.
In the following three photos, 3 students are standing spaced apart: Person 1 is 3 feet from the light, Person 2 is 6 feet from the light, and Person 3 is 12 feet from the light. Using a light meter, we measured the light at each person’s spot and took the following three images:
ISO 800, f 4.0, 1/60
Exposed for Person 1
ISO 1600, f 4.0, 1/60
Exposed for Person 2
ISO 3200, f 4.0, 1/30
Exposed for Person 3
Notice how the exposure needed to be increased to let more light in for each person as we moved from farther from the light.
Next, the students were placed in a line with about 1′ between each of them with the closest person 1' away from the light. Notice that the light on Jacob is much brighter than on Ryan.
Compare that to the following image. The students are in the same formation, but placed much farther from the light source.
The light now appears to be much more even across all three students. This is because light falls off dramatically at first and then more slowly as it moves across a distance.
The following two images show the same phenomenon:
The gray cards were placed close to the light source. You can see that the light falls off somewhat dramatically from the front card.
When cards were moved farther away from the light source you can see that the light is much more even across all three cards.
Production stills by Cody Kjos
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