Nikon D300 Complete Manual page 606

subject in the next room, well beyond the doorway. If you
choose to use Auto Aperture or Automatic flash mode, you
need to always watch to make sure the flash sensor is seeing
the same subject as the lens and is not blocked by cables or
other objects.
Manual (Ë): Available with any Speedlight that supports
Manual flash modes, including the internal flash on the D300.
In Manual flash mode, the Speedlight fires at a fixed output
you select. It's up to you to perform the calculations to ensure
that the proper amount of flash is produced. Many Speedlights
have variable power levels, plus their output is also
dependent upon what focal length the flash head is set for,
thus doing manual flash calculations sometimes takes a bit of
time, as well as consulting a Guide Number chart. On the
other hand, a correctly made manual flash calculation should
always provide exactly the right amount of flash on a subject.
The general formula is:
Aperture = GN / Distance
or GN = Distance * Aperture
or Distance = GN / Aperture
Make sure that the GN you plug into those formulas is
expressed in the same units as the Distance (feet or meters),
and that you're using the correct GN for the focal length set
on the Speedlight. Also, make sure that the GN you look up is
for the ISO value set on the camera body (Nikon's flash
manuals all use ISO 100 values; to convert them to ISO 200,
multiply those values by 1.4; for ISO 400, multiply by 2).
Likewise, if you're using less than full power, make sure
you're using the correct GN for the lower power. Most recent
Speedlight models show a distance indicator on their LCD in
this mode, though the limited "resolution" of this indicator
means you can't totally rely upon it.
Flash modes can usually be set on the flash unit, too:
Thom Hogan's Complete Guide to the Nikon D300
V1.02
Page 606