I scan my 4 x 5 negatives, color and b/w using and Epson 3200 scanner. Depending on how careful I am I need to do more or less spotting using the clone tool. I keep finding in an apparently clear sky small out of focus 'puffs' which I've assumed are caused by specks of dust far from the plane the scanner focuses on, perhaps on the underside of the glass. I can't see any such specks on the bottom of the glass, and these puffs always appear in the sky and rarely in other parts of the scene. Dust on the top of the glass appears as white spots with well defined edges. I wonder if what I'm seeing are atmospheric phenomena actually out there in reality. A typical 'puff' is about 20 pixels in diameter in an image at 2160 ppi. That would make it about 1/4 mm on the negative if it is indeed there. I can't see anything like that under high magnification on the negatives but, the contrast level is so low that I wouldn't really expect to see anything.
One wouldn't normally see these 'puffs' in a print unless it were very large, and I haven't yet made such prints.
I have been spotting the 'puffs' also, but if they really are out there, perhaps I should leave them there for verisimilitude.
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