Digital Video Techniques - What is the Inverse Square Law?

When considering how to put together a scene or set dynamic for recording with digital video, you cannot just think in terms of the creative elements of story and characters. Creating effective images has a technical side to them and getting light and objects to play together perfectly can sometimes feel like a scientific formula. When trying to get subjects arranged just right in unison with the lighting devices, sometimes you have to employ what is called the Inverse Square Law.

The Inverse Square Law deals directly with the distance from the light source to the object that is being lit. It states that the more distance that is put between the light and the object the more surrounding objects can be illuminated, but the more the light will be diffused. This means that the light beam that is emitted onto a specific area will be more spread out, and more things will be in its path. In doing so the light becomes softer, and less able to give fast fall-off and strong color contrasts.

This usually needs to be considered when you have a number of people or objects in a given place but limited light sources. This happens often when recording on location while using a portable light kit. You may want to include everyone in a certain location in a given video clip, but you only have one light. What you then need to do is back the light up so everyone is hit by it, and then open the iris more so that you are able to bring more light into the camera. If you do this then it is important that the same amount of diffused light hits everyone, and that there are no other light sources in the room. This is important because any other light source would suddenly become very dominant in the image because of the changes in iris settings.

The diffused light will not be as effective for capturing images, but if you have a high-quality camera it should still be able to secure the image without a lot of light. Then you can go ahead and do light and color corrections in post-production. The saturation, or intensity, of the colors will be lowered once you diffuse the light with distance, but this can also be altered in the editing room.

The Inverse Square Law is something to consider when you have limited resources available to you. No matter what, it is a concrete rule marking the quality and size of a light stream in correlation to its distance from the objects, and it always needs to be planned for when preparing a digital video lighting set-up.