Nissin i40 Instruction Manual page 19

For Nikon Version only
Following functions are available on Nikon cameras. Refer to
camera owner's manual for details.
Slow shutter synchronization
The flash is controlled at a slow shutter speed to the correct
exposure for both the main subject and back ground in low
light conditions or at night.
Red-eye reduction
To prevent the subject's eyes from appearing red, i40 fires
three controlled flashes just before the picture is taken.
Red-eye reduction can be combined with slow sync.
Rear curtain synchronization
In rear-curtain sync., the flash fires just before the rear
curtain closes. By using this function at slow shutter
speeds, a moving subject will appear with such moving
marks behind.
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