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Photoshop® CS4 Bible by Dan Moughamian, Simon Abrams, Stacy Cates

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17.1. Get Rid of Overall Pixelated Look

Noise, that excessive grain in an image, can result from using a high film speed (ISO) setting, from shooting in low-light conditions, or it can be introduced when scanning images on a flatbed scanner. Most point-and-shoot digital cameras produce noisy images with ISO settings above 400 — or less, in some cases. Some current high-end SLR (single-lens reflex) digital cameras, on the other hand, can shoot noise-free up to an ISO of 16.0.

There are two types of noise: color noise and luminance noise. Color noise, which is also referred to as chroma noise, appears as colored blobs that become really obvious when you zoom into the image. Trust me, once you know what to look for, they're all you'll be able to see when you look at images shot with cheap digital cameras. As its name would suggest, color noise is visible in one or more color channels of a photograph — most often the blue one. Click on the Channels panel's tab, or choose WindowsChannels, and click around between the different color channels of the image to examine where most of the color noise occurs. Luminance noise more closely resembles traditional film grain, and as a result, is often more tolerable than color noise.

17.1.1. Reducing noise

Photoshop offers five options under the Noise section of the Filters menu. The first, Add Noise, is the only one that's used to introduce noise ...

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