Not all flashes are created equal. That’s a sheer fact. And their inequality is listed in terms of GN, or Guide Numbers. The GN of your Canon flashes, Nikon flashes, Sony flashes, or Olympus flashes (as well as any other flashes, that is), tells you how far the flash will reach for a specific ISO (usually 100 these days).

So, why is this useful?

Let’s say you have a flash with a GN of 34, which is what the new SB-900 has at its widest zoom. This means the flash can extend for 34 meters at ISO 100, tops. You have to divide the GN by your aperture to determine actual working range. Shoot at f4 and the flash will be good for 8.5 meters at ISO 100 (that’s almost 28 feet for us in the States). You can use this knowledge backwards, too. If you know your flash has a GN of 34, and your subject is, say, 5 meters (around 16 feet) away, then 34/5 = 6.8, which means about f6.3 or 1/3 stop brighter than f5.6.

Next time, extending your GN with ISO.