I'll happily be corrected on this, but my understanding is this:
The dust is not on the sensor proper, but on the AA/IR filter mounted on top of the filter. This means there is a gap between the sensor and the dust.
When you take a photo at a large aperture, out of focus light rays hit the sensor and describe large fuzzy discs (circles of confusion). These hit the AA filter (and the dust) and continue on to the sensor itself, casting a shadow of the dust on the sensor. Because the dust occupies a relatively small portion of this circle of confusion, the shadow is very diffuse and is not noticeable. At small apertures, out of focus light rays hitting the sensor are describing very small circles of confusion, so the dust occupies a much more significant portion of the disc and thus its shadow is better defined.
Cheers,
Peter
I have noticed that dirt shows up more often on photos taken with wide angle lenses at small apertures than with telephotos. Seems like depth of field is playing a roll but the dirt is on the sensor and not the lens. My question is why does lens & depth of field seem to affect the visiblity of sensor dirt/dust?
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