Flash Guide Number by Distance
Returns ideal aperture from flash Guide Number and subject distance.
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Flash guide number and distance
The guide number (GN) is how a flash's power gets described, and it links aperture to subject distance through GN = f-number × distance. Manufacturers quote the GN at ISO 100, usually in meters and occasionally in feet. Rearrange the formula and the working distance falls out on its own: distance = GN / f-number.
Take the Canon Speedlite 600EX-RT, which is rated GN 60 m at ISO 100 with the zoom set to 200 mm. At f/8 the farthest you can still expose well is 60 / 8 = 7.5 m. Push the ISO up and the effective GN scales by √(ISO/100), which means ISO 400 gets you twice the reach.
Applications
Manual off-camera flash. The Strobist-style setups David Hobby made popular. Studio strobes when there's no meter handy, event work in venues you've never shot before, and a quick sanity check for when TTL metering throws you something odd.
FAQ
Do I still need this with TTL? TTL (E-TTL II on Canon, i-TTL on Nikon, P-TTL on Pentax) works out the exposure for you. Even so, knowing the GN helps you pick a flash strong enough for the room and tells you ahead of time when you're about to run out of range.
Why does GN change with zoom? A zoomable speedlight tightens its beam as the focal length grows, so the effective GN climbs. The trade-off is a narrower angle of coverage.
What about modifiers? A softbox, umbrella or diffuser costs you anywhere from 1 to 3 stops of light. For each stop lost, divide the effective GN by √2.
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The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.