Nikon D300 User Manual page 749

▪ ISO 200 sensitivity
Nikon SB-400 Flash Unit GN Information (real values)
▪ GN of 98 (18)
▪ 35mm zoom-head setting
▪ ISO 100 sensitivity
Both of the flash units have the same GN listed, so which one
is really more powerful? The Nikon SB-400 will literally
blow away the Super-Duper unit. Yet, the Super-Duper
manufacturer lists the same GN! The Super-Duper unit must
have its zoom head set to 80mm, a much narrower beam, and
have twice the camera ISO sensitivity to equal the Nikon
SB-400 unit. Mr. Super-Duper is hoping you won't check the
fine print at the bottom of the advertisement so that you'll
think his much less powerful unit equals the Nikon SB-400.
Surprisingly, there are flash unit manufacturers who will do
exactly what I am describing.
What can you learn from this? The actual GN itself is not
enough to make a decision on which flash unit to use. You
must know what the GN is based on in order to make an
informed decision. Take your time when buying a flash unit.
You're safe in sticking with Nikon's Speedlights, since the
ratings are well known and they're designed to support all the
features of your D300(S).
There are also excellent aftermarket flash units available from
manufacturers like Vivitar, Sigma, Sunpak, Metz, Braun/
Leitz, and others. Examine the underlying settings and not
just the guide number itself. What the GN is based on is as
important as the actual number.
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