Sir Isaac Newton takes credit for the origin of the Inverse Square Law, which applies to physical law, such as light, sound, gravity and many other physical objects, including radio waves. In the case of photography, without citing mathematical equations, simply put, when a light source is moved twice the distance away from the subject, the light falling on the subject decreases four times. The light path also expands out over an area four times greater in width and height.
For example, image your light meter reads F/11 when your studio light source is five feet from your subject. If you move the light so that it is 10 feet from the subject, you will reduce the amount of light on the subject to ¼ of its previous power or two full F/stops. To properly expose your image you’d have to open the lens up to F/5.6 (allowing four times more light to enter the lens than when it was set at F/11).
To avoid having to do a lot of math, you can actually use the aperture scale on your camera lens as a cheat. For example, imagine that you start with your light at a distance of 5-feet, 6-inches from your subject. If you move the light back to 8-feet, you will record about a one-stop drop. Move the light further back to 11-feet and your light will drop another stop. The same happens at 16-feet and at 22-feeet. To compensate for this light loss, you would increase the light intensity by one full stop at each interval (8, 11, 16, and 22 feet) or open your lens up by stop at each interval.
Understanding this effect can help you produce the desired amount of contrast on your subject (moving the light away from the subject increases the contrast). It can also help you control the appearance of your background (see page 50 in Rolando Gomez’s Glamour Photography: Professional Techniques and Images). For example, if your background is five feet behind the subject and the main light five feet in front of the subject, the total distance between the main light and the background is ten feet. Because this is twice the distance from the main light to the subject, you’ll know that the light on the background is ¼ the intensity of the light on the subject This ensures that the background will be darker than the subject.
The Inverse Square Law can also be combined with the knowledge of reflectance (see the 90-Percent Rule, page 75 in Rolando Gomez’s Glamour Photography: Professional Techniques and Images) to help you place dark and light subjects fro a balanced exposure (and/or modify the light). For example, when placing two people of different skin tones side by side in a photograph, you would place the darker-toned subject on the main-light side of the frame and the lighter-toned subject on the other side.
Here’s another example: If your subject has platinum hair, you should recognize that it is both highly reflective and that this is in the most intensely lit area of the image (hair will typically be a little closer to the main-light than the skin). This should alert you to the need to block some of the light from hitting the hair in order to keep the exposure of this area in line with the exposure of the skin, which is more important to the final image. To do this, I’ll often have my assistants place black cards around the hair just out of the image frame.