Flash Guide Number by Distance

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.