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Photography & video

Flash Guide Number Calculator

Estimate aperture from a flash guide number, subject distance and ISO setting.

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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 guide number at iso 100, flash-to-subject distance, iso.
  2. Select Calculate to view the result.
  3. Check the method and assumptions below before using the result.

The method, explained

Adjusted guide number = GN100×sqrt(ISO/100). F-number = adjusted guide number/distance.

A WORKED EXAMPLE

Using guide number at iso 100 = 40 m, flash-to-subject distance = 5 m, iso = 200, the result is 11.3137085 f-number. Change these example inputs to match your task; use the method above to check each step.

Understanding your result

Direct flash at the guide number’s zoom setting. Bounce, modifiers, high-speed sync and ambient light are not modeled.

What to keep in mind

Direct flash at the guide number’s zoom setting. Bounce, modifiers, high-speed sync and ambient light are not modeled. 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 guide number calculator?

Estimate aperture from a flash guide number, subject distance and ISO setting. Enter guide number at iso 100 in m, flash-to-subject distance in m, iso. The starting example is editable; use values for the same system or project.

Which assumptions affect this result?

Direct flash at the guide number’s zoom setting. Bounce, modifiers, high-speed sync and ambient light are not modeled.

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