Nikon D750 DSLR Camera (Body Only) - Manual (Page 164)

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If On is selected for ISO sensitivity settings > Auto ISO
sensitivity control in the photo 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
photo shooting menu, 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
(P, S, A, and M Modes Only)