Nikon D810 - Manual (Page 239)

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A
Interval Timer Photography
If interval timer photography is activated before the first exposure is
taken, the camera will record exposures at the selected interval until
the number of exposures specified in the multiple exposure menu
have been taken (the number of shots listed in the interval timer
shooting menu is ignored). These exposures will then be recorded as a
single photograph and interval timer shooting will end (if On (single
photo) is selected for multiple exposure mode, multiple exposure
shooting will also end automatically).
A Other Settings
While a multiple exposure is being shot, memory cards can not be
formatted and some menu items are grayed out and can not be
changed.