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Both Basic and Advanced modes are available. The Advanced mode has an additional tab that lets you apply noise reduction separately to individual channels. That's a valuable capability, because some cameras are particularly poor performers at high ISO settings (ISO 800 and above) in particular channels. If you learn that your digital camera needs noise reduction in, say, the green channel, you can do it with the Reduce Noise filter in Advanced mode.

The basic noise reduction features include sliders for setting strength (the amount of noise canceling applied); how much detail you're willing to lose as the filter removes what it deems to be "noisy" pixels; how much color noise (those multicolor specks you see in noisy digital images); and how much to sharpen detail (which can tend to cancel the noise-removal effects). There's also a checkbox for removing JPEG artifacts, which are little clumps that appear in images, often in the shape of little spiders, as supposedly redundant image information is discarded during JPEG compression.

If you're working with my test image, you'll see there isn't much noise to remove, so we'll move on. However, your own photos may have enough noise to benefit from this filter.

Despeckle

Despeckle is a clever filter that examines the pixels in your image, looking for areas with a great deal of contrast between them. In an image, such areas generally mark the boundaries of edges. When the Despeckle filter finds these edges, it leaves them alone and, instead, blurs other parts of the image. That produces lower overall sharpness in most of your image (the non-edge parts), which tends to blur any speckled areas so the speckles blend in. However, because the edges remain sharp, your image may still look acceptable. The Despeckle filter is a quick way to mask a lot of dust in images that contain many different spots, or to minimize the halftone screen of pictures that already have been printed in a book or magazine. It works best in images that are fairly sharp to begin with, especially those that may have a bit of noise that can benefit from the blurring. It works worst with images that are not very sharp, because the blur effect can be objectionable.

This filter has no controls to adjust. Simply select it from the Filter > Noise menu and see if it does the job. If not, choose Edit > Undo, or Edit > Fade to fully or partially reverse Despeckle's effects.

Dust & Scratches

The Dust & Scratches filter is a smarter tool that actively seeks out areas of your image that contain spots and scratches. Photoshop performs this magic by looking at each pixel in the image, then moving out from that pixel radially, searching for abrupt transitions in tone that might indicate a dust spot on the image. If a spot is found, only that area is blurred to minimize the appearance of the defect. This filter has two controls, shown in Figure 4.11.

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