Flash calculator

Guide number — enter your flash’s GN (from its manual, at the zoom setting it is quoted for), then pick what to solve for: the farthest distance it reaches, the aperture to set, the ISO needed, or the power setting to dial in. A table lists the reach at each whole-stop aperture.

Power — every setting from 1/1 to 1/128 in ⅓ stops, labelled the way flashes show them (1/8 +0.3 is one-third of a stop above 1/8), with the GN and the reach at each. Tap a row to use it.

Falloff — how much darker the background is than the subject (2 × log₂ of the distance ratio, in stops), a chart of exposure against distance from the flash, and the band that stays within ±⅓, ±½ or ±1 stop.

Several flashes — add flashes with their own GN, power and distance: the light adds, so the f-numbers combine as √(f₁² + f₂² + …). Two equal flashes are one stop more (GN × √2).

HSS & ambient — above the camera’s sync speed the flash must pulse (high-speed sync). The page estimates the guide number left, and then compares flash and ambient light at your settings: the shutter speed sets the ambient, distance and power set the flash, and aperture and ISO move both.

Lighting — three parts. Key-to-fill ratio: type how many stops brighter the key is than the fill, or a lighting ratio such as 3 (for 3:1), and see both ways the ratio is quoted. Softbox: how big a softbox or umbrella looks from the subject at a distance, and whether that makes soft light. Light placement: start from a classic setup (three-point, Rembrandt, loop, split, clamshell, flat) or add lights with a role, an angle around the subject (0° is the camera, positive to the camera’s right), a distance and a power in stops. The diagram, seen from above, places them round the subject; drag a light to move it. The page adds up the light reaching each side of the face and gives the resulting ratio, and each light’s level at the subject compared with the key. ⛶ Full screen (or F) gives the diagram the whole screen; Esc leaves.

GN = distance × f-number, GN ∝ √(ISO/100), GN ∝ √(power): the definitions in every flash manual, which follow from the inverse-square law (illuminance from a point source falls with 1/d²).

Exposure value EV₁₀₀ = log₂(N²/t) − log₂(ISO/100) (ISO 2720); the scene list uses the usual EV table (bright sun = EV 15, the "Sunny 16" rule).

High-speed sync is a rule of thumb, not a measurement. Entering HSS costs about 1–2 stops (set it under HSS entry loss) and each stop of faster shutter costs one more stop, because only the light emitted while the shutter slit crosses a point exposes it. The HSS guide number tables in Canon, Nikon and Godox speedlight manuals fall by roughly √2 per stop of shutter speed in this way, but the entry loss differs between flashes — use your manual’s table when it matters.

Lighting ratios follow the convention in portrait-lighting texts: a fill from near the camera reaches both sides of the face and the key only one, so lit : shadow = (key + fill) : fill. A key one stop over the fill is 3:1; two stops 5:1. Some sources quote key : fill (1 stop = 2:1); the page shows both. In the placement diagram, lights within 15° of the camera axis count on both sides, others on their own side; rim, hair, kicker and background lights are left out, and each light’s level at the subject follows the inverse-square law.

Softness is the light’s apparent size from the subject: 2 × atan(size ÷ (2 × distance)). "Soft when the source is larger than the subject and about as close as it is wide" is a rule of thumb.

Bounce and modifier losses are typical figures; a white ceiling 1 m above the head loses about 2 stops, a darker or higher ceiling much more.

Guide numbers are measured by the maker, usually optimistically and at the longest zoom setting; a 24 mm zoom setting can be half the quoted GN. A flash meter or a test shot beats any calculation.

The inverse-square law holds for small sources at a distance. Large softboxes and umbrellas close to the subject fall off more slowly than this predicts.

Several flashes add as shown only when they light the same side of the subject.

The placement diagram is flat (seen from above): it ignores height, so a clamshell’s key above and fill below sit on the same line, and it does not model modifiers' spread, feathering or spill.

TTL and auto-ISO change the flash or the exposure for you; the page assumes manual flash and manual exposure.