Nikon D300 Complete Manual page 626

Note: As described in the Custom Settings section, getting the
right Frequency and Times values for Repeating Flash takes
calculation. Start by first establishing a shutter speed you're
going to use. Let's say your shutter speed is 1/2. Next,
determine the frequency (how many flashes will be fired a
second; Step 1f). A frequency of 10Hz means 10 times a
second, so we need to divide the lower value of our shutter
speed (2) into this, which tells us that we could have as
many as 5 flashes while the shutter is open, which we set in
our repeats value (Step 1e in the above; as you can see
from the steps in setting procedure, Nikon got things
backwards—we'd really want to perform Step 1f first, not
Step 1e).
2. Activate the flash by popping it up (press the Flash
Release button).
3. Set the camera to Single Servo AF; flash only operates
when the camera achieves focus; this step isn't
technically required, but I like to tell people to use it
as a reminder that focus has to be achieved for flash to
operate).
4. Focus on your subject by pressing lightly on the
shutter release. Note the distance on the scale on the
lens.
5. Select an exposure mode. Typically Shutter priority or
Manual exposure modes work best, as we're
constrained by shutter speed due to our calculations
in Step 1.
Thom Hogan's Complete Guide to the Nikon D300
the OK button to select the flash frequency to
be used.
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