Modifiers and Light Falloff

You will probably hear a lot about the Inverse Square Law and how it affects light falloff; the intensity of the light. But it is important to understand that the inverse square law really only applies to a "point light source" and we don't use point light sources. We use and create "collections of point light sources." (See: Modifiers do not make lights larger)

It is true that we can use the Inverse Square Law reasonably accurately *when our light is too far away.* By "too far away" I mean it's a light source where the distance is negating the benefit of modifying it. For a modifier to be truly effective the size of the modifier needs to be equal to the size of the subject and used at a distance equal to it's size (i.e. a 24" softbox used from 24" away to light 24" of subject/area). Beyond this distance our light source acts like a single source and the inverse square law works reasonably well.

The easy way to remember the Inverse Square Law is that changes in distance will change the intensity of the light source exacly as if you had changed the aperture of the lens. For example: If at 2 (ft, meters, whatever) you have a "correct exposure" and you move the light closer to 1.4 you will have increased the intensity by one stop. And if you move the light further away to 2.8 you will have reduced it's intensity by one stop. Or, everytime you change the distance by two (2x or 1/2) you change the intensity by two stops (f/2, f/4, f/8, etc).


Now, I said this was "too far away" and here's why.

In this image I have divided the softbox face up into three areas (it's actually millions of light sources). As you can see, at greater distances the difference in distance to the three sections of the softbox is very similar. Because the distances are very similar we can apply the Inverse Square Law to the softbox as a whole.

But, because the distances are similar we get flatter/lower contrast lighting. And because the angles to the different areas of the softbox are shallower (as seen by the subject) there is less wrap and the light is "harder." There is little benefit to having used a softbox in the first place and further modifications will have little affect (i.e. adding a grid).

The results will be more like this "hard lighting example.

Coworker Emily by Pat David

Coworker Emily by Pat David


Conversely, when our modifier is used at shorter distances we start ganing control. We create lighting with more contrast and wrap with a more pleasant "softness" to it.

In this drawing I have moved the subject closer so that it is inside the effective distance (i.e. the modifier is *larger* than the subject area being lit).

Now we cannot apply the Inverse Square Law to the softbox as a whole because the distances to different areas are significantly different. The law still applies, but it applies to each area. In the drawing the middle area will have one stop less light hitting the subject, and the furthest area will be two stops less (assuming perfectly even diffusion). This gives us contrast and control.

And because the section furthest away is at a much larger angle to the subject, it can "see around" the side...this is what gives us "wrap" and "soft light."

And now, if I want to change the character of the light I can add additional modifications effectively. For example, I can add a grid. What a grid does is to break the surface into smaller groups of light sources and act as a set of barn doors/flags for them. And because the surface of our softbox is actually millions of light sources, a tighter/taller grid is more effective.

Now we have lighting that can be more like this:

Lili by Onny Carr
Lili by Onny Carr

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