Not only Canon flashes, but all brands express the power power of their flashes with the Guide Number. Canon numbers their flashes with the guide number. The guide number is the distance at a specific ISO at a specific zoom head position.

Why is the Guide Number so important?

  1. It allows us to calculate the right exposure. Without the right guide number, no correct exposure. It’s that simple.
  2. It allows us to compare different flashes.
Flash Guide Number Flash Head Position
220EX 72 feet or 22 meters at ISO 100 Fixed position
430EXII 141 feet or 43 meters at ISO 100 flash head at 105mm zoom setting
580EXII 191 feet or 58 meters at ISO 100 flash head at 105mm zoom setting
  • By doubling the ISO, the guide number of the flash increase by 1.4, which is the square root of 2. The 580ex @ ISO 200 and the head fully zoomed at 105mm = 58x1.4 = 81 meters
  • By halving the ISO, the guide number of the flash decrease by 0.7. The 580ex @ ISO 50 and the head fully zoomed at 105mm = 58x0.7 = 40 meters

The guide number is used to calculate the f/stop, therefore the exposure. In the old time we used the slide rule at the back of the flash and now the computer does the job or calculating the f/stop.

f/stop = Guide Number / Distance

and

Distance = Guide Number / f-stop