[Dabhand16]

You only need to know that the higher the GN - the more powerful the flash is, and therefore the longer the range is.

If you are going to use manual mode it also enables the calculation of the apature. The GN divided by the distance that the light travels to the subject - not the range from the camera to the subject which is different if you are bouncing the flash - gives you the apature.

So a flash with a GN of 56 (at ISO 100) with the light travelling 10 feet will result in an apature of f5.6. In the days of film with focal plane shutters this would usually be at 1/30 sec synch speed, but some would go to 1/50.

As the ISO gets higher, the GN will also increase. There would usually be a chart supplied with the flash to give the GN for different ISO (or ASA as it was then).

TTL and intelligent metering makes flash photography much easier. In manual mode you also had to compensate for diffusers, turn the flash down for fill-in or fool it by setting a higher ISO, and in my first use of a flashgun, make sure you had enough bulbs!
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