Nikon D3100 Reference Manual page 88

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.
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Hi 1/Hi 2
Pictures taken at these settings are more likely to be subject to noise (randomly-spaced
bright pixels, fog, or lines).
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The Shooting Menu
ISO sensitivity can also be adjusted using the ISO sensitivity
settings option in the shooting menu (0 132).
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See Also
For information on enabling auto ISO sensitivity control in P, S, A, or M modes, see page 132.
For information on using the Noise reduction option in the shooting menu to reduce noise
at high ISO sensitivities, see page 134. For information on using the Fn button and the
command dial to select ISO sensitivity, see page 146.
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