Nikon D810 - Manual (Page 135)

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If On is selected for ISO sensitivity settings > Auto ISO
sensitivity control in the shooting menu, ISO sensitivity will
automatically be adjusted if optimal exposure can not be
achieved at the value selected by the user (ISO sensitivity is
adjusted appropriately when the flash is used).
1
Select
Auto ISO sensitivity control.
Select ISO sensitivity settings in the
shooting menu, then highlight Auto
ISO sensitivity control and press 2.
2
Select
On.
Highlight On and press J (if Off is
selected, ISO sensitivity will remain
fixed at the value selected by the
user).
Auto ISO Sensitivity Control