Nikon D5100 Reference Manual page 88

If On is selected, photographs taken at shutter speeds slower __ I I /.
than 1 s will be processed to reduce noise (bright spots,
randomly-spaced bright pixels, or fog), increasing the time
required to record images by roughly 1.5 to 2 times. During processing, "Job mr"
will blink in the viewfinder and pictures can not be taken (if the camera is turned off
before processing is complete, the picture will be saved but noise reduction will not
be performed).
Photographs taken at high ISO sensitivities can be processed to reduce "noise:'
High Reduce noise (randomly-spaced bright pixels, lines, or fog), particularly in
Normal photographs taken at high ISO sensitivities. Choose the amount of noise reduction
Low _erformed from High, Normal, and Low.
Noise reduction is only performed at ISO sensitivities of ISO 1600 and higher. The
Off amount of noise reduction is less than the amount performed when I.owis selected
for High ISO NR.
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