Nikon D6 Reference Manual page 700

Flash Modes
The options available vary with the exposure mode.
Option
[Fill flash]
(front-curtain
sync)
[Red-eye
reduction]
648
Flash Modes
Description
This mode is recommended in most
situations. In modes P and A,
shutter speed will automatically be
set to values between
1
/ s with Auto FP High-Speed
8000
1
Sync) and / s.
60
Use for portraits. The flash fires
before the photograph is taken,
reducing "red-eye".
•
A flash unit with red-eye
reduction is required.
•
The desired results may not be
achieved if the subject or camera
moves before the shutter is
released (this setting is not
recommended with moving
subjects or in other situations
requiring a quick shutter
response).
Available
P, S, A, M
1
/ s (or
250
P, S, A, M
in