Originally Posted by Jon Shiu
just to be clear, only the top glass needs to be AN glass.
4x5 seems to stay flat in my D2 and D6.
35mm and to a smaller degree 120, curl and are not held flat unless between glass. You are compromising the enlarging lens by not being flat in that it can`t focus on the center and edges at the same time. Make a sample print of focus on edges and another focus on center and see the difference. The long mirror Peak will allow you to see this without making prints. Then just put the neg between two sheets of small glass ,3x4 will do, and make another print. Mask it down so you don`t get flare all over the place.