Nikon D300 User Manual page 205

Figure 3-45. High ISO NR sample images
Noise in a digital image is somewhat similar. You want pure,
clean images when you take pictures, but instead, you have
some digital noise interfering with the clarity. The
higher you turn the camera's ISO sensitivity, the more the
grainy digital noise degrades your image. The noise-to-signal
ratio can become damaging to the picture. How can you make
the visual noise go away? Use High ISO NR, that's how!
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