Nikon D5100 Reference Manual - Page 172

Long Exposure NR G button ➜ C shooting menu 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), increasing the time required to record images by roughly 1.5 to 2 times. During processing, "l m" 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). High ISO NR G button ➜ C shooting menu Photographs taken at high ISO sensitivities can be processed to reduce "noise." Option High Normal Low Off Description 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. Noise reduction is only performed at ISO sensitivities of ISO 1600 and higher. The amount of noise reduction is less than the amount performed when Low is selected for High ISO NR. i 154