The inverse square law is one of the most important yet misunderstood concepts in photography. On the surface, it basically says that the intensity of a light source will decrease as you move the light away from your subject, but how does that apply to the highlights and shadows in a portrait? In this small excerpt from the "Illuminating the Face" tutorial, Peter Hurley breaks down both the math and the practical application of the inverse square law.
When someone first starts playing around with a camera, they are bound to hear a bunch of catchphrases touted by other photographers. Sayings like "it's all about the glass," "it's the photographer, not the gear," or "use the golden ratio" are all things you will probably hear when you first show interest in photography. One of the more nerdy but useful comments you will eventually hear tossed around will deal with the Inverse Square Law. It is easy to dismiss this law, because, well, photography should be more about art than math, but the inverse square law is one of the most important concepts to understand when it comes to lighting.
The inverse square law basically explains why you see different gradients of light across different distances. If you have ever looked at a white wall adjacent to a brightly lit doorway, you may have noticed how the white wall appears to be different shades of white and gray as you move your eyes farther away from the doorway. The human brain is great at understanding that the wall is all one color. A camera, however, records these transitions of tones in a way that makes it much more obvious to the viewer. Surfaces that are white or black in person might actually appear a different shade in a photograph.
According to Wikipedia, the Inverse Square Law can be summed up as follows: The intensity of light radiating from a point source is inversely proportional to the square of the distance from the source; so, an object (of the same size) twice as far away receives only one-quarter the light in the same time period.
One of the most powerful ways to manipulate this phenomenon as a photographer is to place your lights so that the color or tone of your background changes. Depending on how close you set your strobe to your subject in relation to the background, you can either make your background lighter or darker in appearance. Peter demonstrates this perfectly in the video above, but you can also see it clearly in the GIF below.
In the above example, the background is a simple sheet of white paper. However, when you move the light farther away from the background while still properly exposing the model, you can make the background appear white, eggshell, medium gray, thunder gray, or even black. This is a powerful concept to understand when you need control over your background in a small space or if you want multiple background tones with a single backdrop, such as seamless paper or a painted white wall.
Portrait photographers have another way of manipulating the inverse square law in a way that is much more subtle than changing the background color. In the example below, you can see how different a subject's skin tone can vary simply by the distance the light is placed from their face. If the light is placed close, it appears softer, but the transitions from highlights to shadows happen very quickly. Photographers often refer to this rapid transition as "light falloff." If the light is placed farther away from the subject, the quality of light will appear harsher, but the difference in tone between the highlights and shadows will be a smoother transition.
Understanding the inverse square law and how it affects the scene you are photographing is a critical skill for any photographer to master. When you combine this concept with other concepts like the size, shape, and material of your light modifier, the possible ways you can light a portrait are endless.
If studio photography and portrait work are things you are interested in learning more about, check out Peter Hurley's full 4.5-hour-long tutorial, "Illuminating the Face". I've included the promo video below so you can see all of the topics discussed in this digital download.
A lot of people have left comments on Youtube or below saying that they are having a hard time understanding the difference between how different shadows are hard, soft, even, or dense. Using such words can be difficult to understand when you are just getting started so I've tried to simply it even more. Keep in mind, this video does not directly explain the difference between hard light and soft light which is caused by small and large light modifiers, but since we essentially did vary the size of the light modifier in the 2nd half of this video, I thought it would be interesting to make a graphic showing how everything relates.
In the above example taken directly from the video lesson, I think you can clearly see how moving the light further away from your subject makes the shadow transitions softer and less dense. If you move the light source closer to the subject, you can easily see how the shadows become softer, they have a less defined edge, but the shadows also become much more dense.
Obviously you could change the light modifier on the left hand image to something HUGE and FAR AWAY and you would get soft edged shadows that are also less dense. Similarly, you could change the light source on the right hand image to something SMALL and VERY CLOSE and you would get a sharp edged shadow that also is very dense.
So there is most definitely a relationship between modifier size and the sharpness/softness of the shadow edge. In this example though, we wanted to show you how the contrast between shadow and highlight decreases as you move your light source away and increases as you move the light source closer to your subject. I hope that makes things even easier to understand. If you want the absolute best scientific explanation on how light behaves in photography, check out the book Light: Science and Magic: An Introduction to Photographic Lighting. This book is amazing and explains how light behaves in very technical and scientific ways.