What is the Inverse Square Law?Updated 2 hours ago

What is the Inverse Square Law in Photography?

The inverse square law explains how light intensity decreases as the distance from the source increases. In photography, this means light falls off rapidly the farther it travels.

In simple terms, when you double the distance from a light source, the light doesn’t just become half as bright; it becomes one-quarter as bright. This principle applies to all point light sources, including strobes, speedlights, and constant lights. Understanding this helps photographers predict how light will behave across their scene and maintain consistent exposures.

How Does the Inverse Square Law Affect Light Falloff?

The inverse square law causes light falloff to be more noticeable close to the subject. As the distance increases, the change in intensity becomes less dramatic.

For example:

• Starting a subject 2 ft from a strobe and then moving the subject to 4 ft reduces the light to 25% of its original power. (a 75% loss).
• If we do the same movement but start the subject at 10 ft away from the light, moving to 12 ft, this reduces light only slightly (about a 31% loss). Since the change in distance is small relative to the total.

Does the Inverse Square Law Apply When Using Softboxes or Modifiers?

Yes, the inverse square law still applies with modifiers, but the apparent effect is softened because modifiers spread and diffuse the light source.

A bare strobe head acts like a small point source and shows very fast falloff. Adding a large softbox increases the size of the light source, reducing harsh shadows and spreading the light more evenly. While the physics remain the same, the perceived transition from highlight to shadow is much smoother, making the light feel less “dramatic.”

How can I use the Inverse Square Law to Control Light in Portraits?

By adjusting the distance of your light, you can dramatically change the look of a portrait. Closer placement creates rapid falloff, contrast and an increase in power, while further placement softens transitions.

To use this practically:

• For dramatic portraits, place a strobe close to your subject for deep shadows and highlights.
• For more even lighting, move the light further away to reduce falloff and balance the exposure across the scene. Consider using a larger modifier, like an umbrella, to maintain softness on your subject. 
• If you find yourself in a situation where you need more power but your flash is already shooting at full power, remember that moving it closer to your subject will increase the power. 
  The opposite is true if your flash is firing at its lowest power and you need even less. Move it further away to decrease the power. 

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