Quote Originally Posted by Bob Carnie View Post

When you focus the image you are focusing on the grain of the negative which is at a different plane than the glass the neg is sitting on.
I do not think that optical verses regular glass would make much difference... but I have never tested this and would be surprised to find it made a difference.
If it makes a difference then optical glass would be decisive consideration for the bottom piece.

I use AN glass for the top and this I am sure is not in play.
Hi Bob, I agree the top glass should not be in play regarding optical vs regular glass. Also agree some of the imperfections in the bottom glass itself might be out of focus (assuming a large enough aperture). I'm trying to get my head around whether inferior quality and/or uncoated glass below the negative can have adverse optical effects though, since technically it is an additional lens element in the total optical "system" between the neg emulsion and the paper. To have zero effect I assume it would have to be "optically flat" (to optical standards) and free of flare. I don't know, I'm probably making too much of this.