How this calculation works
Guide number links aperture and direct-flash distance for a stated ISO and flash coverage. It is not a distance by itself and cannot be transferred between metre and foot conventions without conversion.
Effective GN = ISO100 full GN × sqrt(ISO/100) × sqrt(power fraction); ideal distance = effective GN/f-numberA worked example
A full-power GN40 at ISO100 becomes effective GN20 at one-quarter power. At f/4, the ideal direct-flash distance is five metres. The inverse calculation for that same five-metre target returns a 0.25 full-power fraction. Raising ISO to four hundred doubles the effective guide number under the ideal square-root relation.
Good to know
Direct normal-flash ideal geometry only; no bounce, modifier, HSS, TTL, recycle time or exposure guarantee. Confirm the guide-number measurement convention and actual coverage. The result is not a flash-device rating or a promise of even illumination.
Reference: Sony — guide number and aperture-distance relation (opens in a new tab) ↗
What this calculation answers
Guide number links aperture and direct-flash distance for a stated ISO and flash coverage. It is not a distance by itself and cannot be transferred between metre and foot conventions without conversion.
This page uses a full-power ISO100 guide number in metres, then scales it for entered ISO and output-energy fraction. The square-root power relation follows the ideal inverse-square exposure model.
The Exposure value and ND filter time calculator handles continuous-light equivalent exposures. Flash GN and synchronization require their own assumptions, so an ND exposure time is not a complete flash calculation.
Use the right input conventions
Use a guide number for the same head zoom or coverage as your scenario. A headline maximum GN measured at a narrow beam angle may not represent a wide-angle setting or a diffuser.
Power is a fraction such as 0.25 for one-quarter output. The target distance supplies a separate inverse question: how much full-power fraction the aperture and distance would require under the same ideal assumptions.
Interpret the result
Required power above one is displayed as beyond full-power arithmetic rather than silently clamped. That makes an unreachable target visible. A target inside the arithmetic range is still conditional on the stated direct-flash assumptions.
Bounce, softboxes, umbrellas, high-speed sync, near-field illumination and automatic TTL metering do not follow the same simple GN input unchanged. The calculator does not estimate those transmission or timing effects.
Work through the example
A full-power GN40 at ISO100 becomes effective GN20 at one-quarter power. At f/4, the ideal direct-flash distance is five metres.
The inverse calculation for that same five-metre target returns a 0.25 full-power fraction. Raising ISO to four hundred doubles the effective guide number under the ideal square-root relation.
Where the model stops
Direct normal-flash ideal geometry only; no bounce, modifier, HSS, TTL, recycle time or exposure guarantee.
Confirm the guide-number measurement convention and actual coverage. The result is not a flash-device rating or a promise of even illumination.
A few questions, answered
Why does quarter power halve guide number?
Guide number scales with the square root of output energy, so sqrt(0.25) equals one-half.
What does required fraction above one mean?
The entered target exceeds the ideal full-power combination at that aperture, ISO and guide number.