Electronic Flash - Guide Number.

GUIDE NUMBER
As you get further away from any light source, the intensity of it reduces (drops off).
As you change the f-stop within a lens, the intensity of the light at the film plane (or digital sensor) also changes.

They change at the same rate, so this statement is always true:
distance x f-stop = a fixed number (Guide Number)

Every combination of FLASH and film speed has one guide number (GN). Faster films and more powerful flashguns have higher guide numbers. Often manufacturers use the GN as a part of the model name labelling - eg Canon 430EX has a GN of 43

You figure out what f-stop to use by dividing the guide number by the distance from the flash to the subject.

When the film plane sensitivity is increased by a factor of 4, the guide number is doubled. So if the GN for ISO 100 is 80, the GN for ISO 400 will be 160.

How film speed changes Guide Number

Film speed:	Guide Number
25	40
50	56
100	80
200	110
400	160