Background Exposure Techniques with Single Light Studio Portraits

What you’re going to learn in this video is one of the most important aspects of photography. This video sums up what is known as the inverse square law. It’s one of the fundamental laws used in not only photography but in a variety of other areas of physics as well. Daniel Norton demonstrates how, using this law, you can create several looks as well as control the background, all with just a single light:

What is the inverse square law in photography?

The law suggests that for every unit of distance that you move a source of light away from a subject, the intensity of the light drops by the square of the distance.

For example, if a source of light is 2 feet away from a subject and the exposure is f/8 at 1/400 of a second, moving the light source to 4 feet will require you to push the exposure by two stops (doubling and then doubling again) to compensate for the loss of light. Meaning you would have to shoot at roughly f/4 to compensate for the loss of light.

In this example, Norton changes the distance between the light and the background, which is a white wall, and demonstrates how the increase in distance between the source of light and the wall affects the color of the wall.

For all the shots he uses a simple 2 x 3 foot softbox with a grid.

For the first shot the light is about 1 foot from the subject and about 4 feet from the background:

subject to background distance

In the second shot, the light has now moved to about 11 feet from the wall. But the subject hasn’t moved. In the first shot, the wall appeared greyish. But in the second shot, the wall appears white.

inverse square law

This happens because light fall-off (diminishing intensity of light) is maximumized at the initial distances. In other words, as the ratio between the subject to light and background to light changes, so does the adverse effects of light fall-off.

One more change you will notice between the two above images is that the second one has more pronounced shadows. This happens because the source of light has now become smaller relative to the subject’s face and thus is now technically a hard light.

For the next shot, the light and the subject have been moved farther away from the wall. Light to subject distance remains just 1 foot while the light to background distance increases vastly. This makes the background become darker while the face is properly exposed.
background to light distance

For this shot the light moves farther away from the subject, but the subject remains at the same spot as in shot #3. The background now appears much brighter, though the strong shadows give away the fact that the light is now hard.

light fall off tutorial

If the light moves even farther away (with the subject remaining at her position), the background should appear perfectly white. Again, this happens because the ratio between the light and the subject and the light and the background is almost the same.

understanding inverse square law

So what happens if the subject is say 20 feet away from the background and the light is just a foot away from her?

subject to background distance

Did you guess right?

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