Nikon D300 User Manual page 754

Front-Curtain Sync
In Front-curtain sync mode (also known as Fill flash in Nikon
litereature), the camera tries its best to balance the light if
you're using a lens that has a CPU in it (see Figure 12-3).
Older non-CPU lenses cause the camera to ignore the ambient
light completely and use only the flash to expose the subject.
A CPU lens, like a Nikkor G or D lens, balances ambient light
and light from the flash equally and works to make the light
look very natural. If you use this correctly outdoors, it will be
hard to tell that you were using flash, except for the catch
light in your subject's eyes and the lack of damaging
shadows. The flash simply fills in some extra light without
overpowering the ambient light. In a situation where there is
very little ambient light, the camera will use only the flash to
get a correct exposure. It only balances with ambient light if
there is enough ambient light to balance with in the first
place.
Figure 12-3. Front-curtain sync
There is a side effect to using this mode with slow shutter
speeds. Front-curtain sync simply causes the flash to fire as
soon as the front shutter curtain is out of the way and before
the rear shutter curtain starts closing. If there is some ambient
light and the shutter speed is long—like 1/2 second—and the
subject is moving, you'll see a well-exposed subject with a
blurry trail in front of it. The flash correctly exposes the
754