What is the inverse square law in photography?

It states that light intensity falls off with the square of the distance: doubling the distance reduces intensity to one-quarter (–2 stops), halving the distance quadruples intensity (+2 stops).

Why does moving the flash closer have such a dramatic effect?

Because intensity scales with 1/d², small changes at close range (e.g. 0.5 m → 1 m) are far more dramatic than the same physical move at greater distances (e.g. 5 m → 5.5 m).

Does this account for light modifiers or zoom settings?

No — modifiers (softboxes, grids) and zoom affect the spread of light but not the inverse-square fall-off rate, which depends only on distance. Use this tool for the distance component, then adjust power for any modifier loss separately.

What unit should I enter for distance?

Any unit works as long as you are consistent — both distances must use the same unit (meters, feet, or centimeters). The ratio is dimensionless.

How do I use the stop change to adjust my flash power?

Most flashes are calibrated in stops (full/half/third stops). If the calculator shows –1.5 stops (less light at the new distance), increase flash power by +1.5 stops to compensate, or open aperture by 1.5 stops.

Inverse Square Flash Calculator

Name: Inverse Square Flash Calculator
Availability: InStock
Author: Nham Vu

Enter your current flash distance and exposure, then see how intensity changes at a new distance — stops gained or lost, f-stop adjustment, and power compensation.

Flash Distance Inputs

Distance Unit

Original Distance (d₁)

New Distance (d₂)

Current Aperture (optional)

Enter to get a suggested new aperture.

Enter distances and click Calculate to see results.

Intensity Ratio

—

new / original

Stop Change

—

+ brighter / – dimmer

Exposure Adjustments

Flash power compensation —

Intensity factor —

Distance ratio (d₁/d₂) —

Fall-off Table from d₁

Distance	Intensity	Stop Change	Bar

Summary

Enter your current flash distance and exposure, then see how intensity changes at a new distance — stops gained or lost, f-stop adjustment, and power compensation.

How it works

Enter the original flash-to-subject distance.
Enter the new distance you want to move the flash to.
Optionally enter your current aperture (f-stop) for a direct new aperture suggestion.
Read the intensity ratio, stop change, and suggested aperture or power adjustment.
Use the chart to see intensity fall-off across a range of distances.

Use cases

Determine the new f-stop after repositioning a strobe or speedlight.
Calculate how much flash power to add when moving a light farther back.
Understand the dramatic light fall-off when using a bare flash at close range.
Plan a multi-light setup by predicting relative intensities at different distances.
Teach or study the inverse square law in a photography or physics class.
Quickly verify whether a new flash position will over- or under-expose the subject.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu