Speedlite Tips & Tricks - Inverse Square Rule of Light

Peggy Farren 2014-04-19

by Joe Fitzpatrick

Recently I had the pleasure of attending a presentation by Booray Perry at the Southwest Florida Professional Photographers Association monthly meeting. Booray is a great speaker and his presentation touched on a variety of photographic subjects. One thing he mentioned that rarely gets enough attention is the Inverse Square Law of Light.

The inverse square law describes how light decreases as you move farther from the light source. The law states that light decreases by the square of the distance. A subject twice as far away receives 1/4 of the light (2 X 2 = 4), one 3 times as far away receives 1/9 of the light (3 X 3 = 9) and so on.

Photographer very close - back row is a little dark.

So what, you say. My TTL flash still delivers the right amount of light. A problem arises when you are photographing a group and they are in several rows. The first row gets the correct amount of light, the second row less and a third row still less. Let's say it is three feet from the flash to the front row and another three feet to the back row. The back row, being twice as far from the flash receives only 25% as much light, a two stop difference!

So how do you even out the light between the front and rear row? Move farther away! Increasing the difference between the distance from the flash to the front row and the distance between the front and back rows lessens the difference in light reaching the rows.

Farther back better proportion
Photographer back 12 feet: Light is better proportioned.

If we increase the distance between the flash and the front row to 12 feet, the distance to the back row is now 15 feet, 1 1/4 times farther way from the flash rather than twice as far. This reduces the light difference between the rows to a manageable amount. ~~~~~~ Peggy's notes: Joe wrote the article and said he didn't have pictures to go with it. I knew I have 9 people at my flash class so I told him I would provide the pictures. There are a couple of side lessons going on in these two images so I want to point them out.

First of all, I want to apologize to my models. I only took one shot each (I should know better) so I have no choice but to put the blinkies in.

Next I want to talk about distortion. You'll notice on the first image where I was really close, the people in the front look huge and the people in the back are smaller. The background has converging lines up toward the ceiling. In the second picture, I was back about 12 feet and zoomed in. The people are all properly proportioned.

The next thing I want you to notice is how the background looks bigger. When you zoom in, you "compress" the entire image so it seems like the background is bigger. The converging lines are now straight lines as well.

Joe Fitzpatrick is one of that rare breed of photographers who possesses both a vast array of technical knowledge and the ability to communicate it in clear, simple terms. Joe and Peggy Farren run Understand Photography Training Center.

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Peggy Farren is an award winning, professional photographer, instructor, writer and speaker.

With over 17 years as a full-time professional photographer, Peggy offers photography training through her training center, “Understand Photography”.

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