A Guide to Portrait Photography, Step Five: The Laws of Lighting

The larger the light source, the softer the lighting effect; the smaller the light source, the harsher the lighting effect.

How to select, set up, and use studio lighting is probably one of the most confusing aspects of studio photography for the beginner. It doesn’t have to be. Selecting the right studio lights for the job and setting them up in the right configurations is relatively easy if you understand the basic laws of light. OK, relax. I’m not talking about the physical laws of light that you may or may not have studied in high school physics class, so there’s no reason to drop my class here. I’m simply talking about a few common laws of light that anyone can comprehend and apply to their photographic experience. We are going to investigate three of those common laws of lights in this article. Those three laws are the following:

1. The larger the light source, the softer the light: the smaller the light source, the harder the light.

2. The closer we place the light to our subject, the softer the lighting effect; the farther the light is from our subject, the harsher the lighting effect.

3. The closer we place the light to our subject, the brighter the light.

The First Law states that the softness of studio lights is directly proportional to their actual physical size or to their apparent physical size, which is why we employ “softboxes” or “umbrellas.” We’ll talk about these devices a little later in this series. Simply stated another way, the larger the apparent light source, the softer the lighting effect, which eliminates harsh edges from around our subject. The edges become soft and fuzzy so they blend gently in with the backdrop. All that you have to remember here at this point is that Big equals Soft and Small equals Hard lighting. That’s simple enough, right?

The Second Law states that the softness of studio lighting is inversely proportional to the distance the light source is positioned from our subject. Softness decreases with distance. Apparent light size is also dependent on the ratio of the actual light size to the size of your subject. For example, a naked strobe light would be a very small light source when used to illuminate a human subject but a very large light source when used to illuminate a Honeybee. As you can see by now, this stuff isn’t rocket science.

The Third Law of Light tells us that the intensity of light diminishes rapidly with distance. Try to stay with me here because this is going to be the most difficult part of our discussion today. Light diminishes rapidly with distance and not in a proportional way. In physics, this is known as The Inverse Square Law. If I was writing an article on the physics of light, I’d go on and explain the Inverse Square Law in great detail, using numerous formulas, but we are photographers so I’ll give you the 30-second explanation of the Inverse Square Law. The Inverse Square Law tells us that if we double the distance we reduce the light intensity to ¼. If we increase the distance four times we reduce the light intensity to 1/16 its original intensity. In other words light intensity = 1/D2 where D equals the distance from the subject. Here’s another way of looking at the same principle. If you need to reduce the light by two f-stops, double the distance of the light from the subject. If you need to increase the light by two f-stops, cut the distance by one-half.

Studio lights are expensive but in the next part of this series, I will show you a way to get a studio lighting system on the cheap.

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carol roach
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Posted on Sep 10, 2009