Ah, the mystery of the out of focus dust particles. This is not as likely, but I have one very short throw c-mount cinema lens for 16mm film that have a couple pieces of dust in between the elements. The dust is normally not visible unless I stop the aperture all the way down to f/22 or f/32, and the area of the frame is light and smooth, like a clear sky. Again, a totally functional lens that I really love how it renders light in all but that one situation.