In the previous chapter we looked at edges, areas of rapid contrast organic to an accurately rendered photograph. Now we look at noise, areas of rapid contrasts that weren't actually there. More specifically noise is random fluctuations in luminance or color between neighboring pixels. The noise was not part of the actual scene that the camera captured. The camera added the noise in its attempt to evaluate or enhance light. Noise is to digital images what grain was to film.
The Sharpening filters from the previous chapter can't distinguish between real edges and noise. As they draw forth the edges, which are good, they draw forth the noise, which is bad. Noise lies in wait ready to wreak havoc. Noise is like a virus that infects edges. Noise disguises itself as an edge, but it is in fact the edge's evil twin. Fortunately, Photoshop arms you with commands designed to defeat noise, reduce noise, Median, Dust & Scratches, and the Blur filters all offer unique ways to defeat or partially resolve noise.
You can even exploit noise to make an image look better. These movies explain how.
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