I just got a Mamiya 645 45mm lens for use on my 1000s. There is a black speck (larger than pinpoint), inside the lens, that is apparent when looking through the lens at a light source. Since I have an adapter for it, I mounted the lens on a DSLR (Pentax K10D) and shot some pictures of white paper, with some lines to give me something to focus on. I did it so that the paper fills the whole frame. Looked at the image(s) blown up and brightened, on the computer screen and can't find the speck. I remember reading that some lens issues don't effect picture quality; is this that type of thing?
If in fact, this is not an issue, is it possible that it doesn't show up using the DSLR, but might on the 1000S?