What is the inverse square law?

The inverse square law states that light intensity is inversely proportional to the square of the distance from the source. Double the distance = quarter the intensity (2 stops less light). Triple the distance = one-ninth the intensity. This applies to point light sources in free space.

How does this affect photography?

Understanding light falloff is crucial for flash photography, portrait lighting and group photos. A flash 2m from a subject gives 4x more light than at 4m. In group photos, people at the back receive significantly less light than those at the front, causing uneven exposure.

Does the inverse square law apply to all light?

It applies to point sources and near-point sources (flash, bare bulb, small softbox at distance). It does not apply to collimated light (lasers, sunlight — effectively parallel rays due to the enormous distance). Large soft sources at close range also deviate from the inverse square law.

How can I get even lighting on a group?

Move the light source further away. At 1m, the difference between 1m and 2m is 75% light loss. At 5m, the difference between 5m and 6m is only 31% loss. Distance flattens the falloff. Using multiple lights or a large reflector also helps even out illumination.

PhotoCalcs

Inverse Square Law Calculator — Light Intensity vs Distance

Calculate how light intensity changes with distance using the inverse square law.

Distance 1

Initial distance

Distance 2

New distance

Intensity at Distance 1

Light intensity (any unit)

How We Calculate This

The inverse square law governs how light intensity decreases with distance:

Inverse Square Law

I2 = I1 x (d1 / d2)²

Stops Difference

Stops = log2(d2² / d1²) = 2 x log2(d2 / d1)

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Last updated: February 2026

All calculations are estimates based on standard optical and photographic formulas. Results may vary with specific equipment.