A measure of the power of a flashgun expressed in metres or feet.
- ‘Such technical items as ‘guide numbers’ don't seem to matter any more.’
- ‘Unless you're shooting under those conditions, consider the guide numbers to be estimates.’
- ‘You can use a flash with coverage wider than the lens you are using with no change to the guide number.’
- ‘The guide number will enable you to calculate the correct aperture, simply divide the distance to the object by the guide number of the flash.’
- ‘Small flashes have guide numbers ranging from 45-80, while large shoe-mount units will be up to around 130.’