Inverse-Square Law Light Falloff Calculator

Frequently Asked Questions

Does the inverse-square law apply to sunlight?

No. The sun is so far away that its light is effectively collimated (parallel rays). Moving a few metres closer or farther from a subject makes no measurable difference in sunlight intensity on Earth.

How is this different from a guide-number calculation?

Guide number relates flash power to aperture and distance. The inverse-square law models how existing light intensity changes with distance - it applies to any light source, not just flash.

Why is the real falloff sometimes different from the calculated value?

Large modifiers, bounce surfaces, and nearby walls all add fill light that reduces the actual falloff. The formula gives the physics baseline for an idealized point source in open space.

Can I use this to match two lights at different distances?

Yes. Calculate the intensity at each distance and adjust the farther light's power (or aperture/ISO) by the stop difference the formula gives you to achieve equal output at the subject.

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Estimates for informational purposes only.

Important Disclaimer: Estimates for informational purposes only.

This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How It Works

Light from a point or small source spreads outward over a sphere. As the radius doubles, the same light is spread over four times the area, so intensity falls to one quarter: I2 = I1 x (d1 / d2)^2. This is the inverse-square law. In photographic stops, the falloff is: stops = 2 x log2(d2 / d1). Moving from 1 m to 2 m loses exactly 2 stops; from 1 m to 4 m loses 4 stops.

The calculator also inverts this: given a target stop change (positive = dimmer, negative = brighter), it solves for the distance that produces it: d = d1 x 2^(-stops / 2). Enter the intensity and distance at your starting reference point, the new distance, and the desired stop change.

This law explains why subjects near a window blow out while the room stays dark, why portrait backgrounds go several stops darker than the lit subject, and how to use distance to balance or separate studio lights.

Use Cases

The inverse-square law is a practical tool across studio and location photography:

Setting key light distance to produce a desired background-to-subject contrast ratio

Separating a subject from a backdrop by moving the light close so the background falls off rapidly

Achieving even group lighting by moving the light far away, reducing the relative distance difference across the group

Understanding window light: a subject 1 m from a window is lit 4 stops brighter than a subject 2 m away

Planning multi-light setups where each source must be positioned for a specific output balance

Tips

Move a key light very close to a subject to create dramatic, fast-falling light and a dark background - even without a black backdrop.

Move a light farther away to even out illumination across a group, because the relative distance change from front row to back row becomes smaller.

A large softbox close to the subject behaves less like a point source, so real falloff is gentler than the formula predicts near the modifier.

The inverse-square law assumes open space; reflective walls and ceilings add bounced fill that flattens falloff in practice.

Always use a consistent reference point (usually the front face of the modifier or the flash head) for both d1 and d2 to keep the math accurate.

FAQ