Michael - I generally use AN glass both above and below the film when printing large format color.
The nature of the dye cloud detail is not affected provided you have the correct type of AN glass
relative to format. With most black and white films I use it only on top, or not at all if the film has
some classic retouch "tooth" to it. Some black and white films are much "slicker" than others.
Theoretically, the glass below the negative could act as a lens and re-focus and/or distort the light path.
But unless that piece of flat glass is really poor (centuries old window glass?) I wouldn't expect that the results of the re-focusing or distortion would be substantial.
One of the things rarely mentioned is how condensation forms much more readily on uncoated glass than coated. A darkroom is typically a damp place, and enlarger bulbs heat the surface very quickly. Uncoated glass attracts atmopheric smudges much more easily too, just like uncoated camera filters; you have to clean them way more often. Secondary reflections from uncoated glass can sometimes be seen using a grain magnifier. Even the top glass can have an unwanted
effect in this respect. I never understood why people will spend top dollar for yet another taking
lens they rarely use, but then throw it all away attempting shortcuts in the darkroom.
Michael - we are in basically the same climate zone, if you find a difference I would like to know.. Hope my clients don't come back asking for a return policy because I used the regular glass from Focal Point.
Drew- Anti Newton glass on bottom??? with no ill effect, now thats a new one on me.
Same question here re anti-Newton ring glass on the bottom. That's a totally new one to me. On the other hand I've never done color enlarging.
Bob: I doubt you'll be issuing any refunds. A nice thing about the multi-resistant coated (B+W "MRC") filter glass I installed in my 35mm carrier is how easy it is to keep clean (as Drew alluded to). That coating is extremely hard too, so it reduces the chance of me buggering it up with a scratch or something. It will last extremely long. I like this stuff for 35mm because with the bigger enlargement factors every little flaw can become more of an issue. Unfortunately since I threw away the regular glass for that carrier a long time ago, I can't test to see if any of this makes any difference. On balance I'd have to agree with you it is likely just overkill in the end, particularly when it comes to enlarging sheet film.
Drew: I agree sometimes people fail to consider the entire end to end system from taking lens to print. That's why I thought this would be an interesting discussion. On the other hand this particular issue might be trivial. I don't know.
Re newton rings, my Tri-X 320 spacer works like a charm. But if I could find the right anti-newton glass I'd surely use that instead. But I only know of Focal Point. So if it is as complicated as you say (ie need different types for different formats), yikes. I wouldn't even know where to start.
Appreciate everyone's input on this so far. Very interesting.
Yes, I routinely use AN glass on the bottom for chromes and color negs. But in each instance I've
carefully selected the specific type. And I do routinely use longer than "normal" focal length lenses
and diffused light sources, so this affects the angle of incidence of the rays. For smaller originals
(4X5 down) the wavy pattern Omega and Durst style glass works quite well. For 8X10 the Focal
Point product is fine. Newton rings are a contstant problem in this climate (classic SF Bay fog). In
many cases I need to use AN glass below slick black and white sheet films like TMX, TechPan, or
Delta 100. A tricky proposition with small 120 or 35mm work, however. It's a delicate game getting
not getting too much MTF out of a high-end enlarging lens in such cases and revealing a bit of the
AN pattern with high-contrast papers. That's why I have kept some of my older regular rodagons
on hand, besides the Apo Rodagon and Apo Nikkors. Tricky in theory to predict all this, but easy to
cumulative test per various applications.
A tougher glass would really be nice as one does stand the chance of scratches over multiple cleanings.
I have about 5 sets(top and bottom) ready to go right now thanks to Focal Point, but I have a monster sheet required for my 11x14 . if this MRC glass is as good as you state then I need to find a source to cut and bevel.