Long exposure nr (long exposure noise reduction), High iso nr, Long exposure nr – Nikon D800 User Manual
Page 301: Long exposure noise reduction)

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If On is selected, photographs taken at
shutter speeds slower than 1 s will be
processed to reduce noise (bright spots,
randomly-spaced bright pixels, or fog). The
time required for processing roughly
doubles; during processing, “l m” will
blink in the shutter speed/aperture displays
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 noise reduction is not available during movie
recording.
Photographs taken at high ISO sensitivities can be processed to
reduce noise.
Long Exposure NR
(Long Exposure Noise Reduction)
G button
➜ C shooting menu
High ISO NR
G button
➜ C shooting menu
Option
Description
High
Reduce noise (randomly-spaced bright pixels, lines, or fog),
particularly in photographs taken at high ISO sensitivities.
Choose the amount of noise reduction performed from High,
Normal, and Low.
Normal
Low
Off
Noise reduction is performed only at sensitivities of 1600 and
higher. The amount of noise reduction is less than the amount
performed when Low is selected for High ISO NR.