Inverse Square Law of Light
Professor R. Warnock
June 14, 2011
What is an “inverse square law of light” Is there a mathematical formula that can explain this inverse square? What happens when the object being photographed is too close to the camera or too far away? What are some examples of the inverse square law of light of 1D at 10, 14, and 20 feet? Does science have anything to do with photography? One just points and clicks the camera, correct? Hopefully this reading will be able to define and answer these questions to clarify them a bit. First we will start with what “inverse square law of light” signifies.
“The inverse square law also known as ISL explains the f/stop numbers…” (Robinson, 2010) Put into simple terms, light intensity that is farther out from the original light changes or in other words, gets brighter and dimmer as it moves away from say a light bulb for an example. As we get farther away from the light bulb, we know that we will be able to see less and less. The same law applies to light sources when taking photographs as it would for getting farther away from the light bulb example. The formula for finding this number is the equation equal to GN (guide number) divided by distance = f/stop. Using a GN flash of 120, the example would look like this: 120/11’ = f/11 or put into simpler terms, 120 divided by 11 feet equals f/stop 11(Robinson, 2010). As one can plainly see without even realizing it, there are science and mathematics in photography!
The intensity of light at 1D and 10, 14, and 20 feet from the flash are as follows;
1D/10’ = 3.16
1D/14’ = 3.74
1D/20 = 4.47
When thinking about a flash being too close to the subject, it conjures to mind the image of a brightly colored blob in the middle of the picture. Sound familiar? Think about Aunt Bess in that old family photograph that all you can make out is her outline or the out of focus image that vaguely resembles Rover with a big sniffing nose taking up the whole image. When flash is too close, the images turn out extremely bright and when flash is too far away, your images can appear dark. This is caused by the camera image being distorted much the same an image would appear to the naked eye if there was too much or too little light surrounding the subject.
The same rules apply to photography as do things in real time moving objects, speed, velocity, size, and weight will all have an impact on photographic imagery. In conclusion, people will not often think about the rules of science influencing an image however, after reading this document, hopefully it has become more clear that science has a direct impact. In fact, you may have a love for science that you never realized before in the way of photography! Of course, the modern technology of cameras has made it quite easy for people to enjoy.
Robinson, E.M., 2010. Crime Scene Photography (2nd ed.) chapters 3 (pgs. 58-60) & chapter 5 (pgs. 240-241). Elsevier Inc., London.