Manual flash, without the guessing
Guide number arithmetic that accounts for ISO, power fraction and the light your modifier eats — solving for whichever of aperture, distance or guide number you do not know.
Guide number calculator
Nothing leaves this device
The relationships
guide number = aperture × distance (at ISO 100)
GN at other ISO = GN₁₀₀ × √(ISO ÷ 100)
ISO scales by a square root because the guide number is a distance and light falls off with the square of distance. Four times the ISO doubles the reach, not quadruples it.
Power fractions and modifier losses are stops of light, and for the same reason they apply to a guide number as a factor of √2 per stop. Quarter power is two stops, which halves the guide number.
Guide numbers are optimistic. They are quoted with a bare head, at a stated zoom position, often measured in a small light-coloured room whose walls contributed to the reading. Treat the result as a starting point for a test frame, not as a specification.
Why the inverse square law rules everything here
Move the flash twice as far away and you lose two stops, not one. That single fact explains most of what is strange about flash: why a group photo has a bright front row and a dark back row, why a flash that lights a room at 3 metres does nothing at 10, and why moving a light 20cm closer to a portrait subject changes the exposure noticeably.
It also explains the most useful trick in flash work. Close to the subject, the fall-off is steep — so a light just outside the frame gives you dramatic separation from the background. Far away, the fall-off across the same subject is gentle, giving flat, even coverage. The distance is the lighting decision; the power is just bookkeeping.
What eats your light
Umbrellas cost roughly 1.5 stops for a shoot-through, more for a reflective one that scatters light past the edges.
Softboxes cost about 2 stops, sometimes more with a second internal diffuser. The bigger the box, the softer the light and the more you lose.
Bouncing off a ceiling is the most expensive — 2 to 3 stops depending on height and colour, and a coloured ceiling tints everything. It is also the cheapest way to make on-camera flash look like real light.
Zoom head position changes the guide number substantially. A head zoomed to 105mm concentrates the light and reads far higher than the same flash at 24mm — manufacturers quote the flattering figure.
Related
Exposure comparator for the ambient side of a mixed exposure, and macro calculator for the light lost at close focus, which flash calculations must account for separately.
Questions people actually ask
What is a guide number?
A single figure describing how much light a flash puts out, defined so that aperture × distance = guide number at ISO 100. A GN of 60 in metres means f/6 at 10 metres, or f/12 at 5. It is the manufacturer's way of quoting power in photographic units.
Why does doubling the ISO not double the reach?
Because light falls off with the square of distance, so a guide number scales with the square root of ISO. Four times the ISO doubles the working distance; doubling ISO gains about 40%.
My flash never reaches its stated guide number. Why?
Guide numbers are measured under flattering conditions — bare head, zoomed in, often in a small light room whose walls added to the reading. Real distances also include any modifier, and a softbox alone costs about two stops. Expect to need one to two stops more power than the specification implies.
How much light does a softbox cost?
Roughly two stops for a typical box, one and a half for a shoot-through umbrella, and two to three for bouncing off a ceiling. Larger modifiers cost more because they scatter light over a wider area. The field above takes any of these in stops.
Does flash exposure depend on shutter speed?
Not for the flash itself — the burst is far shorter than any normal shutter speed, so the whole thing lands within the exposure regardless. Shutter speed controls the ambient light around it, which is exactly how you balance flash against daylight. Aperture and ISO affect both.