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Canon EOS 40D Guide to Digital Photography by David D. Busch

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Custom Function II (C.Fn. II): Image

C.Fn II-01: Reducing Noise Effects at Shutter Speeds of One Second or Longer

Long exposure noise reduction

Visual noise is that awful graininess that shows up as multicolored specks in images, and this setting helps you manage it. In some ways, noise is like the excessive grain found in some high-speed photographic films. However, while photographic grain is sometimes used as a special effect, it’s rarely desirable in a digital photograph.

The visual noise-producing process is something like listening to a CD in your car, and then rolling down all the windows. You’re adding sonic noise to the audio signal, and while increasing the CD player’s volume may help a bit, you’re still contending with an unfavorable ...

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