Understanding Light with the Inverse Square Law

Lighting for photography can be difficult to master for many people. The light we have the most human experience with is the sun, which is quite large and powerful. In the space of the universe it is very close to us, but on a human scale 93,000,000 miles is a great distance. I mean, how far can we see, in daylight, through our atmosphere? To give or take 100 miles from the sun’s distance is insignificant. When we work with our cameras and people in daylight, this light source can be treated as a constant, which is why the Sunny 16 Rule has worked quite well since the early days of photography. But it also might be why we don’t necessarily have an intuitive understanding of the Inverse Square Law.

The Inverse Square Law as it relates to photography is just the observation that as light travels from where it starts, it’s intensity will decrease proportionally to the inverse of the square of the distance. So for example, say in a dark room with no ambient light you set up one light two feet from your girlfriend on her right side and set your camera for a proper exposure. Easy, right?

1-light-1-person

 

The light is about two feet from your girlfriend; so far so good. Now add her mother to her left side (camera right).

1-light-2-person

As you can see the distance from the light source to your girlfriend’s mother is roughly four feet, or double (x 2) the distance from the light to your girlfriend. You might expect, therefore, that she would be getting half as much light as your girlfriend, but you would be wrong. She would actually get only 1/4 of the light. The Inverse Square Law states that if you double the distance (x 2) you get the inverse of the square (2 squared is 4, and the inverse of that is 1/4).

1-light-3-person

So now you add your girlfriend’s father another two feet to camera right. Remember, your girlfriend is two feet from the light, her mother is four feet, and now dad is six feet from the light (or x 3 the distance from the light to your girlfriend). That means that he is only getting 1/9 of the light your girlfriend is getting (3 squared is 9, and the inverse of that is 1/9). Time to move the light or add another light source!

As you can see, adding people to any photograph that you are lighting makes things more complicated. You have to make sure everyone is lit evenly (unless you have a creative reason not to). This can be done in a number of ways. The first thing you can try is arranging the people and the placement of the light so that they are roughly equal in distance.

Another option, if you are using a light that can be directed or focused such as a strobe/speedlight, is to “feather” the light – so using the last diagram with three people you could aim the light toward the person farthest away so that the greatest concentration of light would go there and the closer people would get less concentrated light, but being closer the light would even out.

A third option is to add another light source, such as another light or a reflector. Usually, in my experience, it is a combination of all three techniques that yields the most pleasing results. Once you understand how the Inverse Square Law works lighting becomes a whole lot easier and more intuitive.

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