The One Equation That Runs Every Flash
Before TTL metering, before through-the-lens anything, photographers exposed flash with a single equation: GN = distance × f-number. The guide number is a fixed property of your flash at full power and ISO 100; divide it by the flash-to-subject distance and you get the aperture for correct exposure. A GN 60 (meters) speedlight, subject 5 meters away: 60 ÷ 5 = f/12 — set f/11 and shoot. It is not an approximation or a rule of thumb; within the limits of the inverse-square law, it is physics.
Why bother in the TTL age? Because TTL meters the reflected scene and gets fooled by white dresses and black tuxedos, while guide-number math meters the light itself and never lies. Manual flash with GN math gives repeatable, consistent exposure shot after shot — the reason studio and event photographers still live by it.
Guide Numbers: Meters, Feet, and Zoom Heads
The first trap is units: guide numbers come in meters (GN 60) or feet (GN 197) — same flash, different numbers, related by ×3.28. Always confirm which your manual quotes, and never mix a meters-GN with a feet-distance. The second trap is the zoom head: a speedlight's GN changes with its zoom setting, from maybe GN 28 at 24mm to GN 60 at 200mm, because the head concentrates the beam. Use the GN for the zoom setting you are actually shooting — or better, set the zoom manually and note its GN from the manual's table.
Manufacturers quote GN at maximum zoom and ISO 100, which flatters the spec sheet. Your real-world number at 50mm zoom is considerably lower — one more reason to verify with a test shot rather than trust the headline.
The Inverse-Square Law in Plain English
Light falls off with the square of distance: double the distance, quarter the light (two stops). This is why GN math divides rather than subtracts, and why small distance changes matter hugely up close — moving a flash from 1 meter to 1.4 meters costs a full stop, while moving it from 10 to 10.4 meters costs almost nothing. Close-up flash work demands precision; distant flash is forgiving. It is also why bouncing flash off a ceiling costs so much light: the path is flash→ceiling→subject, often tripling the effective distance, which is 3+ stops gone before modifiers enter the picture.
ISO: The Free Lunch
Guide numbers are quoted at ISO 100, but effective GN scales with the square root of ISO: ×1.41 at ISO 200, ×2 at ISO 400, ×2.83 at ISO 800. A GN 60 flash becomes effectively GN 120 at ISO 400 — that is two free stops of reach with zero extra gear. The trade is noise, but modern sensors make ISO 800–1600 a perfectly reasonable flash-shooting range. When you are GN-limited (big room, high ceiling bounce), raising ISO is almost always smarter than buying a bigger flash.
Modifiers: Paying for Softness
Every modifier eats light — that is the price of softness. Typical losses: softbox 1–1.5 stops, shoot-through umbrella ~1 stop, reflective umbrella ~½ stop, beauty dish ~⅔ stop, grid ~1 stop (plus the beam-narrowing effect). In GN math, each stop of loss divides effective GN by √2 (≈1.41): a GN 60 flash through a 1.5-stop softbox behaves like GN 42. This calculator applies the modifier loss before solving, which is the step formula-only guides skip — and the step that explains why your softboxed exposure came out dark.
Distance still dominates: doubling the softbox distance costs two stops on top of the modifier's own loss. Big softbox close to the subject is the soft-light recipe precisely because proximity beats power.
Power Fractions: Stops You Can Dial
Manual flashes offer power in fractions — 1/1, 1/2, 1/4, down to 1/128 — where each step is exactly one stop. The conversion is simple: needed fraction = (target aperture ÷ full-power aperture)². Full power gives f/11 at your distance but you want f/5.6? (5.6/11)² ≈ ¼ — set 1/4 power. The helper above does this live. Beyond exposure, fractions control two precious resources: recycle time (1/4 power recycles in a fraction of a second versus several seconds at full) and battery life (hundreds of pops at 1/8 versus dozens at full). Shooting at the lowest fraction that does the job is simply professional.
Putting It Together: A Real Workflow
Here is the manual-flash workflow that never fails: (1) set your GN from the manual at your zoom setting; (2) estimate flash-to-subject distance; (3) pick your modifier and ISO; (4) solve for aperture with the calculator; (5) set the flash to the nearest power fraction via the helper; (6) take a test shot and nudge ±⅓ stop. Total time: under a minute. After a few sessions you will find yourself doing it by feel — but the calculator stays honest when the room changes.
Multiple Flashes and Ratios
GN math extends naturally to multi-light setups: compute each flash independently, then think in ratios. A key at f/8 and a fill at f/5.6 is a 2:1 ratio (one stop apart) — classic portrait lighting. Two equal flashes at the same distance double the light (one stop): GN_total = GN × √2 for identical units. Mixing brands is fine as long as each unit's GN is honest; when in doubt, meter or test-shoot the combined result, because reflector designs differ.
When GN Math Breaks Down
Honest limits: GN math assumes direct flash in open space. It breaks with heavy bounce (compute the total path and add 2–3 stops of bounce loss), with high-speed sync (HSS pulses the flash and costs 2+ stops — check your manual's HSS GN table), at very close distances where the flash tube's size matters, and with TTL auto modes doing their own thing. In those cases, use GN math as the starting point and the test shot as the truth.
TTL vs. Manual: When Each Wins
Through-the-lens metering fires a pre-flash, meters the reflection, and sets power automatically — brilliant for run-and-gun situations where distance changes every frame: weddings, events, journalism. But TTL meters reflected light, so a bride in white against a dark church fools it into underexposure, and a groom in black fools it the other way; pros ride the flash-exposure-compensation dial constantly. Manual with guide-number math wins wherever the setup is stable: studio portraits, product shots, real-estate interiors, multi-light scenes. The light never changes unless you change it, so exposure is identical frame to frame with zero hunting. Many photographers use a hybrid: GN math to establish the baseline ratio, then TTL for the key light when subjects move. Knowing both systems — and when to switch — is what separates flash owners from flash photographers, and the calculator above serves both styles equally well.
Putting It All Together
Guide numbers turn flash from mysticism into arithmetic: one equation, corrected for ISO and modifiers, solved for whichever variable you need. Set up above — GN, distance, modifier, ISO — read the aperture, dial the power fraction, and shoot. The test shot will confirm what the math already told you, and after a while you will barely need the calculator at all. That is the point: tools that teach.