Flash Exposure
Find the aperture or reach your flash can manage from its guide number.
About Flash Exposure
Guide number, distance and ISO combine to give the aperture needed for correct flash exposure, and the equation reverses to show reach. Flash exposure fails as distance grows, so the guide number equation predicts the working range before the shot is taken.
Formula
Flash Exposure
f-number = guide number / (distance x sqrt(ISO/100))
- guide number
- Flash output rating equal to f-number x distance at ISO 100
Worked example
With the default values below, Flash Exposure returns Required Aperture of 4.00. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Guide numberminimum 1
- 40
- Distanceminimum 0.1 m
- 5 m
- ISOminimum 25
- 400
Results
- Required Aperture
- 4.00
- Reach at f/2.8
- 7.14 m
- Note
- Higher ISO multiplies effective flash range
Step by step
- 1Values used
- 2Flash Exposure
- 3Required Aperture= 4.00
- 4Reach at f/2.8= 7.14 m
- 5Note= Higher ISO multiplies effective flash range
Assumptions and limits
- The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is Flash Exposure calculated?
f-number = guide number / (distance x sqrt(ISO/100)) Guide number, distance and ISO combine to give the aperture needed for correct flash exposure, and the equation reverses to show reach.
Why does Flash Exposure matter?
Flash exposure fails as distance grows, so the guide number equation predicts the working range before the shot is taken.
What values do I need to enter?
This calculator takes 3 inputs: Guide number, Distance, ISO. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.