Flash Guide Number Aperture Calculator
Flash Guide Number Aperture is intended for the specific photography task described by its name. Enter guide number at given iso, flash-to-subject distance m to calculate a result expressed in f-number. The formula works entirely in your browser and needs no account or external API. Guide numbers depend on zoom, ISO and ambient conditions. This estimate is useful for comparing scenarios when the supplied measurements follow the stated assumptions.
How to use this calculator
- Measure guide number at given iso in the units indicated on the input label.
- Enter Flash-to-subject distance m.
- Read the f-number result, then compare alternative inputs to evaluate your own situation.
Flash Guide Number Aperture formula
guide/distance (f-number).
- The expression guide/distance combines Guide number at given ISO, Flash-to-subject distance m. Follow the units on each input: the answer is expressed in f-number.
- Guide numbers depend on zoom, ISO and ambient conditions.
Worked examples
Worked example: Flash Guide Number Aperture
Use these sample measurements: Guide number at given ISO = 36, Flash-to-subject distance m = 4.
Compute guide/distance.
Using the example inputs, the result is 9 f-number. Use this value to verify the formula before changing the measurements.
Understanding the result
Interpret the output as f-number; it is not a measurement taken directly from a physical system.
Guide numbers depend on zoom, ISO and ambient conditions.
Important notes
Guide numbers depend on zoom, ISO and ambient conditions.
Review the input units, any required legal or engineering rules, and unusual edge cases before acting on this estimate.
Frequently asked questions
What does Flash Guide Number Aperture calculate?
It evaluates guide/distance to produce f-number from the measurements you enter. For the example values shown, the result is 9 f-number.
Which assumptions matter most?
Guide numbers depend on zoom, ISO and ambient conditions.
Can I compare two scenarios?
Yes. Enter another set of measurements in the same units and compare the two outputs.
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This page provides a calculation tool plus explanatory information so you can understand what the result means and when the underlying method may not fit your situation.