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

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Identifying and Dealing with Dust

Sensor dust is less of a problem than it might be because it shows up only under certain circumstances. Indeed, you might have dust on your sensor right now and not be aware if it. The dust doesn’t actually settle on the sensor itself, but, rather, on a protective filter a very tiny distance above the sensor, subjecting it to the phenomenon of depth-of-focus. Depth-of-focus is the distance the focal plane can be moved and still render an object in sharp focus. At f/2.8 to f/5.6 or even smaller, sensor dust, particularly if small, is likely to be outside the range of depth-of-focus and blur into an unnoticeable dot.

However, if you’re shooting at f/16 to f/22 or smaller, those dust motes suddenly pop into focus. ...

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