- The Opemus has a focusing slit, but I do not use it (same as with my stolen Durst 600).
I prefer the grain focuser (easier to use).

- The Calumet bookmark is not working for me, but if it refers to the Kodak polymax frame mounted filters (or equivalent), that was the reason for my question.
One picks the best optical glass filter to place in front of the taking lens and then a fairly cheap plastic/gelatin one to place in front of the enlarging lens?
Well, then an el cheapo enlerger lens would be OK, why a top Nikkor, Schneider, Rodagon?