Here and there I read of people who set their lens to a small aperture while testing their sensor cleanness, supposedly to get the best image of dust speckles. However, the image of the on-sensor dust particles should not, to my understanding, be affected by the sharpness induced by the lens settings. Same is true for dust particle on the lens elements themselves. It makes me wonder - do these people misunderstand the theory of how the optical system work, or am I missing something?