Can grids help you break the law?
If you are using soft grids on a softbox in very close to your subject you might be breaking the law. The Inverse Square Law, that is.
Just about everyone learning how to work with light learns about the Inverse Square Law early on. “The intensity of light falling on a flat surface at right angles to a point source of light is inversely proportional to the square of the distance.” If you double the distance between the light and the subject you get 1/4 the amount of light on the subject. Triple the distance and you get 1/9 the light on the subject.
In practice it isn’t so cut and dried. The law applies to a point source of light—we rarely encounter that in our photography. Simplifying it down, it says that as you increase the distance between a light source and a subject the light spreads out and appears to fall off in intensity very quickly. There isn’t less light, but the light is covering a wider area, so fewer photons are reaching the subject with the balance of them zooming on by past the subject and not contributing to the illumination.
Here you can see me using a light meter to measure the output from a 2x3-foot softbox in close and a bit farther away. Without the grid the light falls off as expected, going from f/6.3 in close to f/5.0 backed away. But when the grid is in place the meter reads the same f/5.0 at both distances. Have I broken the law.
What is going on is that in close the meter sees a small number of very bright grid cells. As you back up a bit, but still in close, the meter sees a greater number of cells, but they are less bright. And this averages out to give the same exposure reading at the different distances. This just happens in close. Once you back away to a point where the meter can see the entire softbox things go back to normal and light falls off as expected.
Let’s take a step back to look at what grids do. Grids are the opposite of diffusion. Grids are baffles that control the spread of light. They increase contrast by blocking the light from hitting walls, floors, ceilings, or anything else in the environment that would become a secondary light source reflecting light into the scene to open up the shadows. They also help control the spill of light onto the backdrop.
So, how does this affect photographs? Normally, without the soft grid, as you move your light in closer to your subject the light gets larger as seen by the subject. We say it gets very soft because it fills in its own shadows and causes the shadow edge transition to spread out. It also leads to a loss in color saturation. With the grid, as you move the light closer to the subject the grid cells occludes light from the edges of the softbox so once the light gets close enough for that to happen it appears smaller as seen by the subject. Then as you move it in even closer its size appears to stay the same and the contrast and saturation remain the same.
You can use the slider on the right to compare the scene with and without the grid to see the change in contrast along with the slight change in color saturation. Both images are lit with a 2x3 softbox positioned 12 inches away from the tip of the mannequin’s nose and both were metered at f/9.0. Although at the same distance, the scene lit with the grids has a harder light quality because the grids made the light appear to be smaller. If the softbox was moved back to a distance where the subject sees the entire box the differences go away.
The downside of soft grids is that they show up in reflections. That could be in the catchlight in the eyes of a portrait subject, as you can see here, or in shiny objects in a still life photograph.
Don’t be tempted to put some diffusion over the grid, though, to try to hide the pattern. The diffusion will completely counteract the effect of the grid.
Thanks again for following along! Send me your studio lighting questions. Or take a look at my updated book, Anatomy of a Studio Portrait.