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THE FEUTEX TOOLKIT

Manual Flash Working Distance

Estimate flash distance at an aperture, ISO and power fraction using a supplied guide number.

Photography workspace● Runs in your browser
01 Your input
02 Your result
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FormulaNominal flash distance = gn*sqrt(iso/100*power)/n; Adjusted guide number = gn*sqrt(iso/100*power)
Sources and model references

Worked example

Inputs

Full-power guide number at ISO 100
40 m
F-number
8
ISO
400
Fraction of full power
0.25

Expected results

Nominal flash distance
5 m
Adjusted guide number
40 m

Load the example in the workspace to reproduce these results. Display rounding does not change the underlying calculation.

How to use this tool

  1. Estimate flash distance at an aperture, ISO and power fraction using a supplied guide number.
  2. Set full-power guide number at iso 100, f-number, iso, fraction of full power in the indicated units.
  3. Calculate and compare nominal flash distance, adjusted guide number.

Try this example

{
  "gn": 40,
  "n": 8,
  "iso": 400,
  "power": 0.25
}

Accuracy and scope

Direct flash approximation; guide number depends on zoom and manufacturer conditions. Bounce, modifiers and ambient light are excluded.

Is my data uploaded?

This tool runs locally in your browser. Input and output are not sent to a processing server. Copy or download your result before leaving the page.