There are all kinds of variables: exact AN pattern, angle of light incidence, type of film relative to light scatter, and of course light source.
What I do is try to keep the plane of focus very shallow, so that only the emulsion is truly in focus. This requires an excellent enlarging lens
and everything being in perfect alignment. But that way any dust falling from above, onto the glass, doesn't even come into focus. So in this respect, glass actually helps the dust problem. Don't underestimate just how many ways dust can get onto things, even loading a carrier. And try something like I sometimes do - making multiple generations of masks and then finally a master printing dupe where a hundred dollars worth of sheet film might be involved, and a single significant bit of dust on anything can spoil the entire project, and you get an idea of true cleanroom discipline. It also explains why I went insane long ago. But spotting afterwards can do that to you too. So take your pick. At least my prints are consistently and completely in precise focus.
