In an ideal world Murray, you would have a fresnel properly matched to a particular focal length of lens, although in the real world, I have not seen this type of precision in ground glass set ups..normally, I recommend measuring the film registration when replacing anything in the optical path, that is taking the measurement of the known focal distance that produces the correct focus. and then set the new fresenl or glass at the same and taking a shot to verify the correct focus plane. Now I know that really this is a bit more precise that most people actually do, When setting up screens. In fact, I am not even sure that most of the companies do it either, in the real world, I don't know that 66.7/1000 makes that much difference, when your talking about depth of field and stop down factors when shooting with LF lenses..if my caluculations are correct, we are talking about 0.017018 millimeter, but I might be wrong, I really have not done the calculations alot as of late...I do know that we have only had one focus problem and this was on an older Rollei Medium Format camera..and have not had any reported problems from the LF guys.

I will have to do some calculations base on the RI of our screens, then I would probably be better equiped to give a better answer. One thing to remember is that different glass fourmulations, have different light transmission properties...based on the mineral content of the substrate..but then again, in the perfect world, you would have a fresnel that was optically matched to the lens and ground glass you intend to shoot it with and change everytime you change lenses to have a completely matched optical setup.

Dave