Manual Flash Exposure Calculator

Understanding Manual Flash Exposure: The Guide Number (GN)

The Guide Number (GN) is the fundamental rating used for calculating manual flash exposure. It represents the maximum distance at which a flash can correctly expose a subject at a specific aperture, assuming an ISO of 100. Using the GN allows you to determine the exact aperture (f-stop) needed for perfect exposure based on your distance from the subject.

1. The Core Formula

The relationship between the Guide Number, the required aperture, and the distance is fixed:

Aperture = Guide Number / Distance

For example, if your flash has a Base GN of 36 (at ISO 100) and your subject is 5 meters away, the required aperture would be 36/5=7.2. This means you should set your lens to f/7.2 (or the nearest practical value, like f/8).

2. Adjusting for ISO (The Effective GN)

Since the manufacturer's Base GN is almost always rated at ISO 100, you must adjust it if you are shooting at a higher ISO (like 400 or 800). Raising the ISO increases the camera's sensitivity, effectively increasing the power of your flash.

This adjustment creates the Effective Guide Number (GNeff​):

Effective GN = Base GN x Square Root of (New ISO / 100)

• Example: If your Base GN is 36 and you set your camera to ISO 400:

  • 400/100=4

  • Square root of 4=2

  • Effective GN = 36×2=72

Your flash's effective power has doubled, and you now use the new Effective GN of 72 in the core formula above.

3. Calculating the Required Aperture

Once the correct Effective GN for your current ISO is determined, the calculator performs the final step: dividing that number by your Subject Distance.

This result is the precise f-stop you should dial in on your lens to ensure your subject is perfectly exposed by the manual flash light. By using this method, you eliminate the guesswork and ensure consistent, accurate flash exposures every time.