There are two types of AN glass--one is textured and one is coated. The textured AN glass is only used above the neg, so it shouldn't impair sharpness. The coated type can be used on both sides of the neg and does not affect sharpness. I have the coated type on my Philips/Paterson enlarger and in one of my contact print frames and the textured type in my Nikon LS-4500AF scanner, and I find the textured glass a bit more effective at reducing Newton's rings.