The Inverse Square Law of Light and other goofiness.

OK, today I am actually feeling better and came up with an idea for the blog. This may be information that a lot of budding new photographers have missed or have been taught in a formal setting without regard for its practical application. You may have heard of the Inverse Square Law of Light. There’s formula for it and everything. Just like any other law of physics. Which makes it pretty boring. 🙂

A summary of the law suggests that the energy (brightness) twice as far from the source (light bulb/flash tube) is spread over 4 times the area and is therefore 1/4 the intensity. This means that 3 times the distance means the light is 1/9th the intensity, and so forth. So, right now you are saying “so what?”, right? Well once we see this in practical terms, we can see how powerful this concept is.

OK, to put this in more practical terms I setup a down-to-earth experiment. I setup an Alien Bee 1600 with the standard 7in reflector on one side of the studio (if you’ve been to my studio, I set it up next to sofa in the sitting area). I turned it up to full power. I used a Sekonic meter set at ISO 800 to meter the light. I laid out a tape measure across the floor over towards the shooting area ( a total distance of 33 feet). I took a test exposure every 2 feet. 

I won’t bore you with all the details, but the results confirm how quickly light falls off as you move away from source, but as you continue to move further away, the amount of fall off slows. For instance,  from 3ft to 7 ft, the fall off changes more than a stop from f/90 to f/45. When we are 13 ft away and move to 17ft away, the light drops from f/22.7 to f22.0 (less than 1/2 a stop).

Ok, the lesson to be learned from this exercise? Light fall off fastest when you are closest to the source and falls off more gradually as you move further away from the source. This is a very powerful notion and can change the way you light individuals or groups. For instance, suppose you are shooting a person in room with a distracting background. You’ve tried moving the subject further away from the background, but is still a distraction. What you can do is move the light source closer to the subject, use a large modifier like a softbox or perhaps a large umbrella. The large light source will provide a softer look, and if the light is close enough, then let the light falloff and hide the distracting background.

I hope this helps you understand light fall off and how it can used to improve your photography.