Nikon D300 User Manual page 759

Now let's look into the Nikon Creative Lighting System
(CLS), which allows your camera to control multiple flash
units in a wireless array.
Auto FP High-Speed Sync
The D300(S) has an additional mode that lets it exceed the
normal flash sync speed of i/250s. This is called Auto FP
high-speed sync mode. Remember how normally both the
front and rear shutter curtains must be out of the way
before the flash fires? Auto FP high-speed sync mode lets
you use shutter speeds all the way up to 1/8000s. At these
speeds, the rear shutter curtain follows the front shutter
curtain so closely that only a traveling narrow horizontal
slit is exposing the sensor at any given time.
When you select a sync speed faster than the normal
1/250s, the camera fires the flash in thousands of short
pulses instead of one big flash. The flash fires in a series of
pulses as the narrow shutter curtain slit moves across the
face of the sensor. The faster the shutter speed, the less
power the flash can manage. You must be able to depend
on ambient light in addition to flash when using Auto FP
high-speed sync mode, especially at higher shutter speeds.
However, this let's you shoot your fast lenses (e.g., F/1.4,
F/2.8) wide open while in direct sunlight, due to the very
fast shutter speed.
You can expose properly with very shallow depth of field
due to a large aperture, even though the light is very bright.
We covered this mode in detail in the chapter titled
Chapter 4, under the sub-heading
Custom setting e1 (D300S and
Flash Sync Speed
D300).
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