Nikon V3 - Manual (Page 204)

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Photographs taken at slow shutter speeds are automatically pro-
cessed to reduce “noise” (bright spots, randomly-spaced bright
pixels, or fog), slightly increasing recording times. Selecting On
increases the amount of noise reduction performed at shutter
speeds slower than 1 s and increases the time required to record
images by roughly 1.5 to 2 times. During processing, a warning
will be displayed 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). In continuous
release mode, frame rates will slow and while photographs are
being processed, the capacity of the memory buffer will drop.
Long Exposure NR