Originally posted by Matt Miller Why focus at infinity? Wouldn't minimum focus distance be better?
Here's the optical theory
The best way to make visible very small items sitting on the sensor (or thereabouts) is to have the light falling on the sensor
1) As close as possible to parallel,
2) As close as possible to a uniform , featureless background
(1) Is achieved by setting to the smallest possible aperture (f/22 or more) and by focusing the lens on infinity.
Focusing on Infinity also helps (2) by blurring out most remainding details in the scene.
The opposite logic applies to avoid seeing the stains.
Larger apertures (below f/16 with the sensor stains I have) or focusing on closer objects will make the stains invisible or nearly so. Both methods cause the light falling on the sensor to arrive at more acute angles, depending on which part of the lens surface captured the light.
For me personally, that's actually most of the real-world photography that I do, which is why I'm still happily using my K-5 unit, until Pentax sorts out the solution.....