Nikon D300 User Manual page 750

For comparison purposes, the GN of the D300(S)'s pop-up
Speedlight is 56 (feet) or 17 (meters) at ISO 200. Nikon's
flagship Speedlight, the SB-900, is 157.5 (feet) or 48 (meters)
at ISO 200. Obviously, the larger external flash unit has a lot
more power and can light up subjects that are farther away.
Technical Guide Number Information
For those technically minded among us, the GN is based
on a specific formula: GN = distance × f/number. It is
based on the inverse-square law, which states that doubling
the GN requires four times more flash power. So, a flash
with a GN of 100 is four times more powerful than a flash
with a GN of 50. The guide number represents an exposure
constant for a flash unit. For example, a GN of 80 feet at
ISO 100 means that a subject 20 feet away can be
completely illuminated with an aperture of f/4 (80 = 20 ×
4) using a sensitivity of ISO 100. For the same guide
number and an aperture of f/8, the light source should be
10 feet from the subject (80 = 10 × 8). Fortunately, your
camera and flash combination are capable of figuring the
correct values for you when you use TTL mode.
Now, let's examine the various flash modes found in the
Nikon D300(S) camera. Since I have no way of knowing
which flash unit you'll be using, I'll write from the
perspective of the pop-up flash. Almost everything mentioned
next applies to the built-in flash and most
mounted Nikon-brand Speedlight units you are using, plus
many Nikon-specific aftermarket flash units.
750
Accessory shoe