Sir Isaac Newton takes credit for the origin of the Inverse Square Law, which applies to physical law, such as light, sound, gravity and even radio waves. In the case of photography, simply put, when a light source is moved twice the distance away from the subject, the light that falls on the subject decreases four times and the light path expands out over an area four times greater in width and height. This information is important for studio photographers to understand.
In the Photography Studio
As an example, if your light meter measures F/11 when your studio flash is five-feet from your subject, and then you double the distance of the studio flash to 10-feet from your subject, you just reduced the light by one-fourth of its power, or two full F/stops. To properly expose your image, you’d have to open the lens up to F/5.6. Every time you decrease an F/stop, you have a 50-percent light increase, so from F/11 to F/8 is a 50-percent light increase and from F/8 to F/5.6 is another 50-percent light increase through the lens, or four times the amount of light allowed into the lens.
The same can be said if you move your subject twice the distance from the background when the subject is half that distance to the light source. And the Inverse Square Law is the same whether your light source is intermittent like flash, or a continuous light source like LED or tungsten lights.
If you place your subject 10-feet from the background and the light source five-feet from the subject, because of the Inverse Square Law, your light intensity on the background is at least a quarter of the light that strikes your subject, and this will ensure your background will be darker than the subject and any foreground.
You can use the Inverse Square Law to your advantage with high-key photo shoots in the studio when you simply move your subject closer to the background than the light source itself. One mistake a photographer can make is to believe the closer the light to the subject, the brighter the background becomes when in fact, it’s just the opposite, you must keep the light source twice the distance from the subject than they are from the background to have a light background.
In a low-key scenario, to ensure a black background stays dark, the distance between your subject and the light source must be at least half or less than half than the distance of your subject to the background. So, in this case, if your light source is five-feet from your subject, your subject must be ten-feet or more from the background. It’s about the distance of the light source to your subject in relation to the distance of your subject to the background.
One approximate method to remember the Inverse Square Law is if you start with your light at five-feet, six-inches then move your light back to eight-feet, you’ll record a drop of one F/stop in light intensity, the same as if you go from F/5.6 to F/8 on your lens. Move the light further back to 11-feet and your light intensity drops another F/stop, or the same as if you go from F/8 to F/11. The same happens at 16-feet, F/16, another F/stop loss and at 22-feet, F/22.
When you take the “approximate” into account to maintain the original F/11 exposure value, you’d increase your light intensity by one full F/stop at each interval of movement of the light source from your subject like the aperture scale on your camera. Once you totally understand the Inverse Square Law, you can put it to use to improve your photos as explained in the examples below.
The Inverse Square Law is very important when you photograph groups with rows of people and to make the math simple and explain this easily, let’s place the light source a foot away from the first row. Thus, the light is approximately two-feet from the second row. If your light source is one-foot from the first row and two-feet from the second row, the second row will be two F/stops darker than the first row.
The distance of the light to the second row of people is twice the distance to the first row and if you take the Inverse Square Law into account and move the light source 10-feet away from the first row of people, then the second row, is now only a fraction of an F/stop darker. In theory, you’d have to place the second group 20-feet away from the light source to have them photograph two F/stops darker when the first row that is 10-feet from the light source.
Skin Tone Differences
The same Inverse Square Law and the 90-Percent Photography Lighting Rule will help you when you place two people of different skin tones side-by-side in a photograph. Basically, you’d place the darker-toned subject closer to the main light and the lighter-toned subject to the fill light. If you only work with one light, place the darker-toned subject closest to the light source and keep the lighter-toned subject toward the back. This is a scenario where you want the light source closer to your subject.
Hair Color Differences
When the hair-color of your subject is blonde, or brighter than their face, the F/stop difference between the hair and the face is compounded by an additional and approximate 1/3 F/stop difference because of the 90-percent rule. It’s these types of scenarios that can cause havoc for a beginning photographer. A more experienced photographer will do several things to ensure the exposure for the entire image is correct.
The first step is to move the light source further away from the subject. Just like with group photography of people, as the light moves further away, the difference between the light striking the top of the hair and face is minimal. However, keep in mind, contrast also increases as you move the light source further away from your subject as the light now becomes more specular.
Another step an experienced photographer will take is to have an assistant hold a black card or reflector, like the Sun-Mover Black-Hole, slightly near the model’s hair and ensure the black reflector stays out of the image frame. This same black reflector will add 10-percent black tone into the lighter hair, thus add detail to the hair.
What the experienced photographer has utilized is the Inverse Square Law plus the 90-percent rule to their favor in the latter scenario. When you understand these two fundamental photography concepts it will help you see light differently than the average photographer and you can thank Sir Isaac Newton for teaching us how distance impacts light intensity.