Light Efficiency

Hey, everybody. Tony and Rob are at Texas School this week with another really great class. One of the topics that came up was that of light efficiency - a topic that may interest anyone who shoots using camera flash or studio strobes.

This was shot with a single light source, overhead and behind the subject, away from camera. Metered at f/8 but shot at f/11 to avaoid over exposure. 

Any light, when it strikes an object, will do one of three things. The light energy can be absorbed, in which case it is converted to some other energy such as heat. It can be reflected or refracted. In most cases, it will do some of all of these things. The reaction most people expect is the reflection.

Again, this is shot with a single light source placed behind the subject, away from the camera. A 3x4 softbox creates a beautiful soft light but the camera was set one and a half stops below the meter reading of f/8 to avoid over exposure.

What most people don’t expect, is that the reflected light changes depending upon the angle in which it strikes a subject. Think of it this way. If you throw a tennis ball straight against a wall, it will bounce back. But it looses a lot of energy as it makes that round trip. But, throw the same tennis ball at the wall at a significant angle, and it’s deflected off the wall with much less energy loss. 

In the case of our photography lights, you’ll find that the light appears to be much brighter if it is behind your subject and has a much broader angle of reflection. The reason is that it loses much less energy. 

For example, if you have a light that is pointing at your subject - let’s say almost next to your camera - and you get a meter reading of f/8, then you’ll get a good exposure using f/8.
But let’s move that light behind your subject so that the angle of reflection is much greater. Even though it may still show a meter reading of f/8, it will be much brighter than when it was placed in front of your subject.