Physics is phun...remember. Ok...question. Shoot the sky with your lens open wide and you'll get the blissful impression that your sensor is clean. Stop down to f16 or 32 or whatever and sit on your digital toches and view the file. For the real masochistic rush auto-level it in photoshop. Why the sharp visibility of the insidious little devils stopped down? Is it something analogous to depth of field in reverse, that is, are the offensive contaminants blurred out with a lens opened up because, in reality, they are out in front of the plane of focus in front of the sensor held out there by the protective glass and AA filter and who knows what other layers? fysics anybody???