Flash Photography and The Law

Did you know that there is a law that  we should be aware  of when we use our flash?

This law applies to ALL flashes; the pop up flash on your camera and  the, usually more powerful, separate flash unit that works off of your hot-shoe.

If you don’t know about this law it could seriously  limit your  ability to create  a photograph.

If  I could, I’d site the statute, but it’s not a law that has any  legal basis .

The law I’m referring to is a law of physics that applies to light, called  the Inverse Square  Law.

As my high  school physics teacher would  have  told you,  ”Sam’s no physicist”, however he probably would be happy that I’ve learned to apply a law of physics on a regular basis to my work as a  photographer.

In brief the inverse law  states that, as the distance between a light source and an object doubles, the intensity of the light reaching the object quarters.

In practical application, if the  light from my flash is  lighting  an object that is  five feet away  with an intensity of  f-8  then, when the light from that  flash reaches ten feet, the intensity of the light has decreased or “fallen off”  to an intensity of f-4.

At  seven and 1/2 feet, the  intensity would be f 5.6

How could the ignorance of  this law seriously  limit your ability to create  a photograph?

As an example, on a basic level,  if you are using  your flash and you are  using  an aperture  of f-16 on your camera, you need to make sure that your flash is  powerful enough to output  f-16 worth of  light at the  distance of your subject.

If your flash is not powerful enough to do this, then your subject  will not receive enough light from the flash and will be underexposed.

You’ll need  to move closer to your subject subject closer or open up your aperture.

As another practical application example, it helps us visualize the photograph before we  make  an exposure.

It was the  application of  the inverse square law that allowed  me  to  compose the above  picture.

A further explanation of how the  above photograph was created  could be found at a past post  titled “Mixing Light Sources and Scheming In Color .”

Learn more about flash photography starting on Tuesday, April 6.

This entry was posted on Monday, January 12th, 2009 at 11:02 am. It is filed under Learn Photography. You can follow any responses to this entry through the RSS 2.0 feed.