Nikon D5100 Reference Manual page 89

Adjust ISO sensitivity (_ 54).
I I Auto I$0 sensitivity control
If Off is chosen for Auto ISO sensitivity control in P,S,A,and M
modes, ISO sensitivity will remain fixed at the value selected by
the user (_ 54). When On is chosen, 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). The maximum
value for auto ISO sensitivity can be selected using the Maximum sensitivity option
in the Auto ISO sensitivity control menu (choose lower values to prevent noise
(randomly-spaced bright pixels, fog, or lines); the minimum value for auto ISO
sensitivity is automatically set to ISO I00). In modes P and A, sensitivity will only be
adjusted if underexposure would result at the shutter speed selected for Minimum
shutter speed (I/2000-I s; in modes Sand M, sensitivity will be adjusted for optimal
exposure at the shutter speed selected by the user). Slower shutter speeds will be
used only if optimum exposure can not be achieved at the ISO sensitivity value
selected for Maximum sensitivity. If the ISO sensitivity selected by the user is higher
than the value selected for Maximum sensitivity, the value selected for Maximum
sensitivity will be used instead.
When On is selected, the viewfinder shows IS0-AUT0and the
information display IS0-A. These indictors blink when sensitivity
is altered from the value selected by the user.
[] Auto ISO Sensitivity Control
Noise (randomly-spaced bright pixels, fog, or lines) is more likely at higher sensitivities. Use
the High ISO NR option in the shooting menu to reduce noise (_ 154). Note that ISO
sensitivity may be raised automatically when auto ISO sensitivity control is used in
combination with slow sync flash modes (available with the built-in flash and optional
SB-900, SB-800, SB-700, SB-600, and SB-400 flash units, possibly preventing the camera from
selecting slow shutter speeds.
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