I could introduce you to this weeks topic by telling you it’s about the Inverse-square law, and I could continue with the definition like this: The Inverse-square law is any physical law stating that some physical quantity is inversely proportional to the blah, blah, blah, blah! Yeah, Google it if you want the definition, because I just want to give you the nuts and bolts.
Along with figuring out lots of things like astronomical distances, radiation, and acoustics, this law is used to explain the “falling off” of light over distance. Here we go:
Imagine you have a light source (flash, lamp, etc.) positioned 1 meter away from your subject. You decide to move that light source to 2 meters away from your subject. How much of the light’s power will hit your subject. 1/2? nope, only 1/4 of the original power.
Basically, the rule is every time the distance is doubled, the intensity is reduced to a quarter of the power. As we learned from above when moved from 1 to 2 meters, we have only 1/4 of the light. If that light is moved to 4 meters it’s now 1/16 of the original power or 1/4 of the previous intensity. The equation is simple, take the distance, square that number and then put a one above it turning it into a fraction.
Light naturally travels in a ever widening pattern. For example, the light from your camera flash will travel in a V-shaped or pyramidal pattern.
The light photons do not become weaker, the beams actually distribute over a wider area. Maybe you could look at it as diluting the light.
So, why is this important for photography?
First, let me give you this tidbit of info. When the light shines in a particular direction, the change in intensity drops very quickly in the first few measurements of distance, then the “fall-off” slows down as the distance increases.
Here’s an example.
Let’s say at 1 meter, you have 100% of the light power hitting your subject. When that distance doubles to 2 meters, the light is now 25%, at 3 meters it’s 11%, at 4 meters it’s 6%; but now at 5 meters the power is 4% and every meter after that, you only see a percentage drop of roughly 1%.
This is important for exposure for a couple of reasons, if you have a subject that is very close to the light source and they’re modeling various poses, chances are they will change their distance from the light. Being so close to the light and changing their distance even by one step can create a huge change in exposure.
Now, you take that same model and place them, say 7 meters from the light, they now have plenty of play, front and back with very little change in exposure. Using this method, you won’t have to check for exposure and change your aperture every time your model moves. You might say, that only takes a few seconds, but those bits of seconds start to add up fast on a shoot. Time is money.
Also, this applies with group photos too, if they’re close to the light and one person is behind another, then obviously someone is out of exposure. Moving the group away to that “roomy” exposure area is your answer.