Nikon D300 User Manual page 748

▪ Temperature of ambient air
In reality, the camera has little to do with figuring the GN
other than providing an aperture f-stop number and ISO
sensitivity. So, how can you decide what GN is best without
whipping out a scientific calculator? Just look at the flash unit
advertisement to see what the GN is based on. Here are the
most important figures:
▪ Flash "zoom" angle-of-view setting, (e.g., 18mm or
105mm)
▪ ISO sensitivity
If you see a flash unit advertised as GN 98, just realize that
this is not enough information to make a decision. In this
instance, the number 98 is the GN. It represents the number of
feet from flash head to subject (98 feet). In countries that use
the metric system, an equivalent GN is 18, which represents
the number of meters from flash head to subject (18 meters).
That number by itself is simply incomplete. Don't buy a flash
unit based solely on a GN like 98 or 100 or 111. Here's why.
Let's think about this for a second. Let's say I were a
manufacturer who was desperate to sell you a flash unit. I
might stretch things a little bit. I might say my Super-Duper
flash unit has a GN of 98 (feet) or 18 (meters), hoping you
won't ask about the settings I used to figure the number. Here
is a comparison of two flash units with a so-called
"comparable" guide number:
Super-Duper Flash Unit GN Information
▪ GN 98 (18)
▪ 80mm zoom-head setting
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