Originally posted by geezer52 I have to admit that much of this discussion is a bit over my rather inexperienced head. So hopefully this won't sound too dumb. But if the spot is only showing up in photos using very small apertures, does that pretty much automatically place the spot between the sensor and the cover glass? When I have dust it is usually evident at all apertures
The original post answers your question exactly.
BTW, the particles are between the sensor cover glass and the AA filter, not between the sensor and the sensor cover glass. Both normally are sealed areas, the latter is already sealed in the chip fab's clean room.
Dust on top of the AA filter (more specifically, on top of the IR filter cleaned by ultrasonic waves) is invisible. You can check this using the equations given in the original post. The intensity is
~N^2/d^2 and if the distance
d becomes large enough, dust disappears for all practical
N.
What is visible on top of the IR filter is dirt, not dust. Large enough to be visible at all apertures and at 1/10" or 2.5mm distance to the sensor.