I recently tested out my first pinhole camera (made from an Agfa Isola) and when I scanned the negatives I noticed that there were lots of black dust on the image, in "sharp" focus. My theory is that this is from dust that somehow has been present on top of the emulsion during the exposure. Never encountered it before, usually I'd have issues with dust on the negative during printing/scanning, but this clearly shows up as blank spots on the negative image itself. Any idea what I could do to avoid this problem in the future?