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. Forget about trying to spot them by peering directly at your sensor with the shutter open and the lens removed. The period at the end of this sentence, about .33mm in diameter, could block a group of pixels measuring 40 X 40 pixels (160 pixels in all!). Dust spots that are even smaller than that can easily show up in your images if you're shooting large, empty areas that are light colored. Dust motes are most likely to show up in the sky, as in Figure 9.7, or in white backgrounds of your seamless product shots and are less likely to be a problem in images that contain lots of dark areas and detail.
To see if you have dust on your sensor, take a few test shots of a plain, blank surface (such as a piece of paper or a cloudless sky) at small f/stops, such as f/22, and a few wide open. Open Photoshop, copy several shots into a single document in separate layers, then flip back and forth between layers to see if any spots you see are present in all layers. You may have to boost contrast and sharpness to make the dust easier to spot.
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