Nikon D3200 Reference Manual page 33

Adjust ISO sensitivity (_ 51).
I I Auto ISOSensitivitycontrol
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 (_ 51). 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 100)). In modes P
and A, sensitivity will only be adjusted if underexposure would result at the shutter
speed selected for Minimum shutter speed (1/2000-1 s, or Auto; in modes Sand M,
sensitivity will be adjusted for optimal exposure at the shutter speed selected by the
user). If Auto (available only with CPU lenses) is selected, the camera will choose the
minimum shutter speed based on the focal length of the lens. Slower shutter speeds
will be used only if optimum exposure can not be achieved at the ISO sensitivity
value selected for Maximum sensitivity.
When On is selected, the viewfinder shows ISO-AUTO and the
information display ISO-A. These indictors flash when sensitivity
is altered from the value selected by the user.
Auto ISO Sensitivity Control
Noise (randomly-spaced bright pJxels, fog, or lines) is more likely at higher sensitivities. Use
the Noise reduction option in the shooting menu to reduce noise (_ 131). 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 the optional
flash units listed on page 173), possibly preventing the camera from selecting slow shutter
speeds.
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