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??
Last edited by ic-racer; 02-14-2008 at 09:51 AM. Click to view previous post history.
While the amount of thought and figuring that went into this is commendable, it's a bit over done. Allot of math breaks down when it hits real world conditions. Diffuse source for contact printing is fine, but you can't see or control dodging or burning. The glass is there not just to press for sharpness, but to assure absolute contact, to prevent Newton rings, which is what you will get, sooner or later without the glass, depending on the negative, humidity, and other factors. An enlarger makes a wonderful light source for contact printing, particularly a dichro head, for split printing. The lens aperture matters not a wit, with a good frame, and a miniscule aperture would do little except give unbearably long exposure times. Use a good quality contact printing frame with clean scratch free glass, and strong springs, throw away the slide rule, and print away.
Last edited by JBrunner; 10-22-2007 at 05:29 PM. Click to view previous post history.
"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."
And how long of an exposure does that translate into for Azo paper?
Interesting study, but I'll keep using a contact printing frame...though I suppose that using a bank of UV tubes 3" from the neg is not too dis-similar to your #2...which is how I print my platinum prints (and explains why only big chunks of crap sitting right on the neg shows up, but rarely any thing on top of the glass.)
you are way over thinking this. Just lay your neg on top of a piece of paper and lay glass over that. Either light source will work just fine. Mr. Brunner has a good handle on the technique.
Wow ... that's an awful lot of work .... my brain is hurting.
For a light source I use a 10watt halogen bulb in a desk lamp about 30" above the frame and for filtration I use coloured plastic report covers above the frame. I use a digital watch for a timer.
I use this setup for both VC and AZO - they just have different exposure times and AZO doesn't need filtration
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Don't suck the fun out of this, Jason and Lee know what they are talking about.
This reminds me of my college days when my professor told me that I always seem to find the hardest way of doing things.
The only caveat I can warn against is I use an anti-reflective glass on the side that touches the film to prevent newton rings.
Best of luck!
The traditional light source for contact printing has always been a plain, ordinary light bulb of sufficient intensity. That was all that was needed in the days when we had silver chloride contact printing paper such as Azo. No collimation or diffusion necessary.
Now that those papers are gone and we are printing on enlarging paper, the light intensity has to be much, much lower, hence, more widespread use of enlargers as the light source. I recently measured the light intensities I frequently use just for trivia's sake. For Azo printing, I used a 120 watt bulb, four feet above the frame. Intensity was 70 foot-candles. Under the enlarger using enlarging paper, a typical intensity is 0.43 foot-candles. That's three orders of magnitude less intensity.
But, other than the inherent tonal characterisitcs of the respective papers, my negatives haven't cried one tear about being printed with a bare light bulb or the collimated condenser enlarger source.
Newton rings can form between any two surfaces that are not in complete contact with each other. Glass to neg, or neg to paper. You most often hear us whining about the glass ones, because that's what we have the most trouble with, as the paper to negative contact is seldom an issue in a frame, because of the give of the paper, and felt. Glass to negative are two fairly hard surfaces, and more prone to the problem in a frame, even under the spring pressure, especially if there is the least bit of moisture on the glass to push a curve in the neg and paper. Roberts anti glare glass is a solution, but I haven't gone that route yet.
Originally Posted by ic-racer
Since the negative contacts the paper, as long as the light hitting the paper is even, it's all the same to the paper, except, of course, in the case of dirty, or scratched glass, where a point source light creates shadows because of the problem glass. But again, no dodging or burning. Glass is cheap. Many if us print with condensor enlargers. There is debate (of course, it's photography), but many persons will tell you they are sharper, and have better contrast. Those of us who use them, know the condensors have to be clean, and scratch free, just like an enlarger lens, camera lens, negative, glass carrier, or the glass in a contact printing frame.
The best contact prints in the world have been made in various closets and darkened workshops with a contact frame, or just a piece of glass, and a bare light bulb, with no math beyond the exposure. The methods for outstanding contact prints won't be found in calculations. You'll find it it your negative, brought out by your practice, patience, persistence, and experience. But all means, experiment, nothing wrong with that. Thats how we get good, but you'll probably find the old wheel still rolls pretty good.
Last edited by JBrunner; 10-22-2007 at 08:56 PM. Click to view previous post history.
Weston used a light bulb with some cloth tied around it--probably just a handkerchief. He could adjust the height of the bulb to vary the exposure.