Dust spots are on the AA Filter and are normally so close to the sensor, and so out of focus that they don't appear in the image. The old trick of shooting at maximum aperture, as close to the bars of a cage or a chain link fence to get them to "disappear" in the picture of the animal inside the cage is a parallel.
Even at f/22 the spots are not really in focus, but I believe it is basically the increased DOF that makes them visible. If you never shoot smaller than f/8 or f/11 you may never see the presence of dust on the AA filter. When I do macros, or shots where I want the foreground and distant background to appear sharp, I often use f/22 or smaller (despite the problems of diffraction), so when I clean the sensor, I have to stop way down on my test shots.
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At first, I was thinking that the increased DOF was the reason too, but then as I thought about it logically, I concluded that could not be the case. The aperture of the lens affects the focus of the image passing
through the lens, and has no bearing on the focus of anything that sits between the back of the lens and the sensor.
Light from an image strikes the lens equally at all points on the surface of the lens. With the aperture 'open', light from a single source point passes through the lens via many paths - some directly through the center of the lens, some via the outer edges of the lens. With the aperture smaller, the light that is hitting the outer edges of the lens is cut off, and the only light passing through is that light from the source that is striking the center of the lens (that's why a smaller aperture reduces the exposure, of course). This light that passes through the outer edges is hitting the sensor at an angle, while the light that passes through the center hits the sensor at close to 90 degrees.
I was going to try to create a diagram to explain this, but found this page (isn't google wonderful!) that shows the concept reasonably well:
[a href=\"http://www.trustedreviews.com/digital-cameras/review/2006/08/17/Digital-Camera-Tutorial-Aperture-Depth-of-Field/p2]http://www.trustedreviews.com/digital-came...pth-of-Field/p2[/url] . It illustrates the angle issue perfectly. If you imagine a spec of dust on the sensor, in example one (wide), the light from any given point is going to hit the sensor from a range of angles, so it's 'blocking power' is reduced. But in example two (narrow), the dust spot is going to block the only light available from the source.
I did further reading, and the best recommendation I came up with (regarding testing the sensor after a clean) was to set manual focus, and completely 'defocus' (blur) the external image (to reduce the amount of distraction in the hunt for real dust specs) - so, you point the camera at a bright object (white wall, sky, etc), intentionally blur the image, set a narrow aperture to get only 'perpendicular' light, then take a shot. I followed this approach and ... OMG ... what a lot of dust I have !!!! I've ordered the sensor swab, fluid, and am about to order the blower and brush!
This page had not only an interesting perspective, but also contains several links at the bottom to other articles.
