Nikon D5100 Reference Manual page 73

A
AUTO
If the mode dial is rotated to P, S, A, or M after AUTO is selected for ISO sensitivity in another
mode, the ISO sensitivity last selected in P, S, A, or M mode will be restored.
A
The Shooting Menu
ISO sensitivity can also be adjusted using the ISO sensitivity
settings option in the shooting menu (0 155).
A
See Also
For information on enabling auto ISO sensitivity control in P, S, A, or M modes, see page 155.
For information on using the High ISO NR option in the shooting menu to reduce noise at
high ISO sensitivities, see page 154. For information on using the Fn button and the
command dial to select ISO sensitivity, see page 165.
z
55