How the guide number works
The guide number (GN) is the product of the distance between flash and subject and the f-number that gives a correct exposure. It is stated for ISO 100 and full power, in metres or feet. With GN 40 and aperture f/8 the distance is 40 / 8 = 5 m, and at 10 m the aperture is f/4. ISO 1230 defines the ISO guide number of electronic flash equipment.
GN = Distance × Aperture
ISO and partial power
The guide number grows with the square root of the ISO speed: ISO 400 doubles it compared with ISO 100. Partial power scales it with the square root of the fraction, so at 1/4 power the guide number is halved. A value in feet is the value in metres divided by 0.3048.
| ISO | Factor on the guide number |
|---|---|
| 100 | × 1 |
| 200 | × 1.41 |
| 400 | × 2 |
| 800 | × 2.83 |
| 1600 | × 4 |
| 3200 | × 5.66 |
| Flash power | Factor on the guide number |
|---|---|
| 1/1 | × 1 |
| 1/2 | × 0.71 |
| 1/4 | × 0.5 |
| 1/8 | × 0.35 |
| 1/16 | × 0.25 |
| 1/32 | × 0.18 |
| 1/64 | × 0.13 |
| 1/128 | × 0.09 |
Zoom position of the flash head
Manufacturers state the guide number for a specific setting of the zoom reflector. The Canon Speedlite 600EX-RT, for example, lists GN 60 (metres) or 197 (feet) at a flash coverage of 200 mm and ISO 100. Use the value that the manual gives for the zoom position you set.
Limits
The guide number applies to direct flash in manual mode. Diffusers, bounce flash and light modifiers reduce the light that reaches the subject, and the surroundings and the subject itself change the result. TTL and automatic modes do not use it. Treat the result as a starting point and check it with a test shot.
FAQ
What is a flash guide number?
The guide number states the power of a flash in one figure: distance times f-number for a correct exposure at ISO 100. A higher guide number means a longer reach at the same aperture. ISO 1230 defines how it is determined.
How do I calculate the distance from the guide number?
Divide the guide number by the f-number. GN 40 at f/8 gives 5 m.
How do I get the guide number from a test shot?
Multiply the distance from flash to subject by the f-number that gave a correct exposure, then divide by the ISO factor (the square root of ISO / 100) and the power factor to get the value for ISO 100 and full power. At 5 m, f/8, ISO 100 and full power the guide number is 40.
How does ISO change the guide number?
The guide number is multiplied by the square root of ISO / 100. ISO 200 gives a factor of about 1.41, ISO 400 a factor of 2.
What is the difference between a guide number in metres and in feet?
The value in feet is the value in metres divided by 0.3048, about 3.28 times as large. GN 60 in metres is about 197 in feet. Distance and guide number must use the same unit.
Does the shutter speed matter for the flash exposure?
Not for the flash light itself, as long as the shutter speed is not shorter than the flash sync speed. A longer shutter speed lets in more ambient light.