Nikon D6 Reference Manual page 420

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Enable or disable silent photography.
Highlight [Silent
photography] and press 2 .
- If [On] is selected, the only time the sound of the shutter
or mirror will be heard is when the mirror is raised or
lowered at the start and end of shooting.
368
Varying Focus over a Series of Photos (Focus Shift Shooting)
Highlight an option and
press J .