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Photography & video

Flash Power and Distance Calculator

Estimate the new flash-power fraction needed after changing flash-to-subject distance.

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Photo & video desk
Photo & video desk

Match the sensor, lens and output settings to your equipment.

See the method

Set up your shot

Runs on your device

Your camera calculation

Your result

Let’s calculate.

Use the Calculate button to see your result.

Review the method below for assumptions and conventions.

How to use this tool

  1. Enter original distance, new distance, original power fraction.
  2. Select Calculate to view the result.
  3. Check the method and assumptions below before using the result.

The method, explained

New power fraction = old fraction × (new distance/old distance)², from inverse-square illumination.

A WORKED EXAMPLE

Using original distance = 2 m, new distance = 3 m, original power fraction = 0.25, the result is 56.25 %. Change these example inputs to match your task; use the method above to check each step.

Understanding your result

Same aperture, ISO, beam and modifiers; point-source approximation. A result above 100% exceeds available flash power.

What to keep in mind

Same aperture, ISO, beam and modifiers; point-source approximation. A result above 100% exceeds available flash power. An ideal geometric or exposure estimate. Lens behavior, stabilization, sensor crop and scene conditions may change practical results.

Common questions

How do I use this flash power and distance calculator?

Estimate the new flash-power fraction needed after changing flash-to-subject distance. Enter original distance in m, new distance in m, original power fraction. The starting example is editable; use values for the same system or project.

Which assumptions affect this result?

Same aperture, ISO, beam and modifiers; point-source approximation. A result above 100% exceeds available flash power.

Methodology maintained by ClarityKit. How these tools are built and checked.