Nikon D300 User Manual page 348

and over as the slit between the shutter curtains travels in
front of the image sensor. The Speedlight can fire thousands
of bursts per second. To a photographer or subject it still
looks like one big flash of light, even though, in reality, it is
hundreds or thousands of bursts of light, one right after the
other.
When you are in Auto FP mode, you'll see something like
this on your Speedlight's LCD monitor: TTL FP or TTL BL
FP.
The FP designation tells you that the camera and Speedlight
are ready for you to use any shutter speed you'd like and still
get a good exposure. Even with wide open apertures!
You can safely leave your camera set to 1/320 s Auto FP or
1/250 s Auto FP all the time since the high-speed sync mode
does not kick in until you raise the shutter speed above the
maximum setting of 1/250 s. Below that shutter speed, the
flash works in normal mode and does not waste any power by
pulsing the output.
This pulsing of light reduces the maximum output of your
flash significantly but allows you to use any shutter speed
you'd like while still firing your external Speedlight. The
higher the shutter speed, the lower the flash output. In effect,
your camera is depending on you to have enough ambient
light to offset the loss in power. I've found that even my
powerful SB-900 Speedlight can only provide enough power
to light a subject out to about 8 feet (2.4 m) when using a
1/8000 s shutter speed. With shutter speeds that high, there
needs to be enough ambient light to help the flash light the
subject, unless you are very close to the subject.
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