Flash guide number calculator
A guide number is the product of aperture and distance at ISO 100. Everything else is a rearrangement.
m
Aperture to use
14.400
- Effective guide number
- 72.00 m
- Guide number in feet
- 236.2
- Reach at f/2.8
- 25.71
- Stops gained from ISO
- 2.000
- Stops lost to power reduction
- 0.000
Show the working
- Guide number at ISO 100 = 36
- Flash to subject distance = 5
- ISO = 400
- Flash power = 1
- iso_factor = sqrt(400 / 100) = 2
- power_factor = sqrt(1) = 1
- Effective guide number = 36 * 2 * 1 = 72
- Aperture to use = 5 > 0 ? 72 / 5 : 0 = 14.4
- Reach at f/2.8 = 72 / 2.8 = 25.7143
- Stops gained from ISO = log2(400/100) = 2
- Stops lost to power reduction = log2(1) = 0
- Guide number in feet = 72 * 3.280839895013123 = 236.22
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- guide number = aperture x distance, at ISO 100
- guide number scales with the square root of ISO and of power fraction
Guide numbers are quoted at a zoom head position, usually 35 mm or 50 mm. A wider setting spreads the same light further and lowers the number.
Manufacturer guide numbers are optimistic. Meter it if it matters.