Flash Exposure Calculator

Automatic flash metering is convenient, but manual flash gives consistent, repeatable results, which is why event, studio and strobist photographers often prefer it. The key is the guide number, a single figure that ties together flash power, distance and aperture. This calculator solves the equation for whichever value you need.

The guide number equation

Guide number equals distance multiplied by f-number. A flash with a guide number of 40 (metres, ISO 100) exposes correctly at 10 metres with f/4, at 5 metres with f/8 and at 2.5 metres with f/16. Guide numbers in feet are about 3.3 times larger for the same flash, so always use the unit stated on the spec sheet.

Three ways to solve

“Find the aperture” takes guide number, ISO, power and distance and gives the f-number. “Find the distance” tells you how far the flash may be from the subject at a chosen aperture. “Guide number needed” tells you how powerful a flash must be for a given distance and aperture, useful before buying one.

ISO and power

Doubling ISO multiplies the effective guide number by about 1.4, the square root of two; going from ISO 100 to 400 doubles it. Reducing power to half divides it by 1.4; quarter power halves it. The calculator shows the effective guide number for your settings.

Moving the light

Light falls off with the square of distance. Move a flash from 2 to 4 metres and the subject receives a quarter of the light, two stops less. The compare field calculates the change in stops for any new distance, which helps when rearranging lights in a studio or repositioning a speedlight.

Bounce flash

Bouncing off a ceiling or wall softens light beautifully but loses a lot of it. Open up about one to two stops compared with direct flash, more for high or dark ceilings. Colored walls also tint the light.

Zoom setting matters

Speedlights advertise their guide number at the longest zoom setting, often 105 or 200 mm. At a wide 24 mm setting, the light spreads more and the guide number drops considerably. Check the table in your flash manual for the zoom setting you use, and enter that number.

Shutter speed and ambient light

Flash exposure depends on aperture, ISO, power and distance, not on shutter speed, as long as you stay at or below the sync speed. The shutter speed controls the ambient light: a slower speed brightens the background behind a flash-lit subject.

Checking results

Take a test shot and check the histogram. Real scenes vary in reflectance; dark clothing and wide rooms need more light, white walls less. Adjust by a third or two thirds of a stop until it looks right, then keep the settings for consistent shots.

Using several flashes

With two identical flashes aimed at the same area, the light doubles, which gains one stop: the effective guide number rises by about 1.4. Four flashes gain two stops. For key and fill lights, set the fill one or two stops below the key by halving or quartering its power, or by moving it farther away.

Related tools

Plan depth of field at your flash aperture with the Depth of Field Calculator. For close-ups with extension tubes, the Macro Extension Calculator gives the extra light needed.