Recently Linda Ralston, one of my Facebook friends, asked me how I go about setting my flash exposure for some nighttime wedding shots I had posted. The conversation on Facebook led to what I dubbed “The Two Immutable Laws of Lighting”. If you can get a grasp on these two concepts you will gain tremendous power over your lighting ability. What are these two laws that will elevate you into wielding the power of light the way a Samurai wields a sword? Continue reading to find out.
1. Light travels in constant directions
Have you even shot pool? If so, you have had an extremely practical lesson in how lighting works. If you aim a light straight, it goes straight, if you aim it into a wall, it bounces off in the opposite direction at the same angle. Before the comments fill up talking about diffusion, refraction, etc, the goal of this is to make the concept easy to understand here.
By understanding this basic concept you can control the angle that light is hitting your subject. This can be used to create soft or harder lighting, change the mood of an image, and create different lighting styles. Again, there is more to this with sub-topics on light spread and reflectivity but the number of variables involved are almost infinite. For the sake of keeping it understandable, you need to keep the concept simple. Light does not follow general guidelines, nor does it follow suggestions, it absolutely follows the laws of physics. Getting a handle of how light functions will give you many, many more options.
2. The Inverse Square Law
“The inverse-square law is a law stating that a specified physical quantity or strength is inversely proportional to the square of the distance from the source of that physical quantity.” – Wikipedia
Pretty heady stuff huh? Let’s break this down so it is as simple as possible to understand, if you double the distance between the light source and the subject, you need four times the amount of light. Ok, simple if you are always doubling the distance, but what if you want to do it fairly quickly? If we need to calculate the light falloff we can take the distance, multiply it by itself, and take the inverse of that number. So if we take a distance of 2 feet, multiply it by itself, we get 4, and take the inverse which would be 1/4.
Putting it Together
Keep in mind that these two things work together in that when you bounce light off something you are adding to the distance between the light source and the subject, therefore you need more light. If the angle causes you to double the distance, you need four times as much light. Often the easiest way to increase or decrease the power of the light source is simply to move it towards or away from the subject.
If you are trying to learn how to shoot with manual flash, understanding these core principles will put you well on your way.