This is a follow up to a question I asked earlier regarding my new VC 28mm Ultron. It arrived with one particularly troublesome bit of dirt that looks like a paint fleck nearly dead center in the lens. I did a test to see if the dirt would affect in the image. I shot a piece of plain white matte board, focused at 3 ft., at every aperture from f2.0 to f16, figuring that any darkening of the image would show up plainly against the white background. The result: I could see no effect at all at any aperture. (I expected that I would see it stopped down, if at all.) As I got a good deal on the lens, I plan to keep it. But I wonder, at what point would dirt and dust inside the lens degrade the image? I would expect that dirt on the front lens element of a macro lens focused close would show up first. Does anyone have any experience or answers? Was my test valid?