I was a little upset today because I spent a few hours going over posts on APUG contact printing forum (up to page 14 of prior posts) and did not find what I need to know to get a good 8x10 contact print.
So, here we go, I have to re-invent the wheel.
Lets start with the basics of light sources . 1) Collimated and 2) Diffuse.
Lets start with a collimated source. The first and obvious question to me is 'do you need an overlay glass' with a collimated source. I believe it is intuitively obvious that a glass is NOT needed if the light source is perfectly collimated (ignoring diffraction around the edge of the silver grains). However, I don't have a perfectly collimated light source available to me for printing.
The closest I could get to a collimated source in my darkroom would be to crank my enlarger to the top of the column and use a very small exit pupil for the light source. Realistically I can get 90mm of column elevation, and I measured an exit pupil of 4mm on a 50mm lens stopped all the way down. Lets assume that the negative won't sit flat on the paper. It is 0.5mm above the paper at the worst area. The question is mathematically illustrated in the JPEG. The solution with these real-life numbers indicates that the circle of confusions formed by the above setup will be 0.022. Thats 10 times smaller than they need to be for most "8x10 at 'normal' viewing distance" calculations (0.2mm).
So, mathematically, you do NOT need a glass when using the enlarger as a light source. In fact using a glass would be quite detrimental in this situation because of the way collimated light would render all the imperfections in the glass.
Now lets consider diffuse light sources. The goal with a diffuse light source would obviously be to minimize scratches and dust on the film base and on the overlying glass. To test the efficacy of diffuse light on the elimination of these pesky scratches I made a simple empiric observation. I took my very scratched and dusty contact printing glass and compared the shadows the scratches made with different light sources. I used my own eyes for the observation. First I observed the shadows cast by the scratches onto a piece of paper just below the glass. The shadows were very obvious!
Then I brought my small light box over the glass and observed the shadows as the light box got closer. When the 8x10 light box was just a few cm from the glass, all the shadows from the scratches and dirt on the glass disappeared! Like magic! (In this observation most irregularities were on the TOP of the glass).
To see how flat the film needs to be pressed against the paper I repeated the above calculations using these values: Exit pupil 300mm, Distance from light source 50mm and circle of confusion of 0.2mm. The gave a result of 0.03mm. (as a comparison I measured some hairs on the back of my hand and got 0.04mm)
So, to answer the question of "how does one contact print an 8x10 negative onto VC paper" I have an answer:
1) Use an enlarger as a light source without an overlying glass
2) Use a diffuse light source (like a translucent white plastic suspended a few CM above the negative, with the enlarger shining on this) and use an overlay glass that can oppose the film and paper to a maximum separation of 0.03mm.
Questions, comments, math errors??