Einstein and Oppenheimer got together and wrote a treatise on the physics of Contact Printing... it was four pages shorter than this topic.
Yes, math errors.
Originally Posted by ic-racer
Your enlarge can only go 90 mm? That's about 3 inches. I'm sure you meant 90 cm, or 900 mm. That makes your incorrect 0.022 result become 0.0022.
I would suggest that your estimate for 0.5mm of separation to be too small. You will probably get at least a couple of millimeters separation as both the paper and the film will have some curvature too them.
Also, you haven't accounted for the scattered light that will spread through your negs as them shine down onto the paper from a few millimeters up. You really should use a glass to hold the paper down to minimze this.
"Einstein and Oppenheimer got together and wrote a treatise on the physics of Contact Printing... it was four pages shorter than this topic."
Just wait until this thread advances to the actual processing of said contact prints.
Why are there no speaker jacks on a stereo camera?
I've been looking at this thread with an increasing sense of incredulity as despite many posters pointing out the simple truth; that contact printing, in principle, is as easy as walking in a straight line, (glass, negative, paper, turn on light), it still appears an impossible quest.
I've only two comments to make which are suggested with the best intentions. As others have said, if the glass is scratched or dirty and leaves visible white marks on the print, obviously buy a new sheet of heavy glass, ideally with bevelled edges or at least with masking tape applied to prevent bleeding fingers in the darkroom. However, if the glass is in top condition and dust keeps appearing, just buy a can of compressed air to blast the offending articles out of your printing vicinity.
My other point, (and I am really starting to realise how boring this post is), is the holy text sought for explaining contacting. OK, Mr Adams kept it brief, because as others have mentioned, there is nothing much to say, (glass, negative, paper, turn on light), but a fascinating resource is the book Darkroom 2, published by Lustrum Press in 1978. The two books in this series simply feature leading American photographers giving an insight into their darkroom practise through folio pages, interviews, and a complete breakdown of the equipment, chemicals and procedures used to shoot, process and print their work. Participants included W. Eugene Smith, Arron Siskind, Ralph Gibson, (founder of Lustrum I think), and most importantly for us here, Cole Weston. Young Cole, son of Edward, has 18 pages to cover contact printing his father's work, and if that's not enough information on the subject, I really don't know what to say. It has wonderful photos of a smiling Cole holding up a roll of toilet paper to illustrate how to clean the glass, and light bulbs for illumination, which puts all ideas of mathematical calculations out of the window. It was that simple all along. But in all seriousness, these books are invaluable resources and give a wonderful insight into how photographers work in the darkroom. Wynn Bullock on tonal print balance and Ralph Gibson on achieving his ideal contrast, (both in the first volume), are marvelous essays. They are long out of print but a quick search on abebooks.com reveall that they can be found quite easily and relatively cheaply.
Thanks for reminding me about this Mike. I shall go away and read that section again
Originally Posted by Mike Crawford
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Mike, i've read those books, they are brilliant and simple
this post, like many others on this site, is just another example of the mis-taken belief that over complicating an issue will make up for lack of experience, confidence and common sense
it don't need to be rocket science
Update and test results.
So, to update, prior to buying a print frame or formal proof printer box, I have ordered some glass from the glass shop. I decided on 10x12, one-half inch thick.
It is a little frustrating waiting for this sheet of glass from the glass shop, so I thought I would try some things.
Q: How much pressure do you need to hold the neg/paper in contact for a sharp image with A) Collimated light and B) Diffuse light.
A: Depends on the paper. I started with the stiffest paper I had, Forte double weight. Room humidity 55%. This paper still curled up ad the edges with 20! sheets of 11x14 glass pressing on a 9x11 area. The foam used for this test was about 15 Shore (A). I had another 20 or so sheets of glass to put on there but it was getting heavy and cumbersome.
I also tried a base consisting of a one-quarter inch sheet of foam rubber about Shore 40 (much stiffer than the first) and it was A) too stiff and B) not flat enough by manufacture tolerances. I did not try the Shore 40 on top of the Shore 15 foam.
Ilford double weight fiber base was held flat under the 20 sheets of glass and the Ilford RC paper also was also held flat.
With this setup I was unable to test collimated light (the light was diffused by all the glass).
I did try a single clear thin piece of framing with weights or pressure on the edges, however, the glass bends and is not in good contact in the center. So, again, I was unable to do this test with collimated light until the thick glass comes in.
So, to recapitulate:
It is impractical to obtain good control with VC paper with a light source consisting of "any light bulb"
Overlay glass needs to be 1) free from bubbles or scratches if one intends to use collimated light. Minimum thickness for a 'gravity pressure' system has not been stated and needs to be determined (for my own work that is).
ANY small imperfection or dust on the glass or negative with show up with precision when using a collimated source.
A diffuse light source removes the shadows from dust and all imperfections in the glass. Dust that gets caught between the neg. and paper still shows up.
If the paper and negative are in close enough contact, both collimated and diffuse light sources produce identically sharp prints.
For a contact printing system to be useful in making fine prints (in my darkroom), there must be provisions for the frequent placement of test strips or paper in the system, without compromising the cleanliness of the negative. Also, when using strips of paper, the negative needs to be in-contact with a clean surface.
For a contact printing system to be useful in making fine prints silver prints on VC paper it has to be designed in such a way that the negative can be loaded on the glass in the light (to facilitate dust removal). The may require inversion of the system or adhesive to attach the negative to the glass.
Now you're getting somewhere. You need to determine pressures with the various thicknesses of papers, film, as well as the printing frame latching system. If you're going to optimize tis system, these numbers need to be quantified. There also need to be a quantificaiton of the pressure variation across the negative/paper laminae on both the X and Y axis. Then we need to know how this pressure varies with the latch spring material be it spring steel, stainless steel or brass. Please include the material grades, heat treatment, typical yield and ultimate strengths. I would also like to see how you determined the spring constants for the various components if you use a mass-spring modeling system.
Originally Posted by ic-racer
As for the light source, glass imperfections, and dust particles, the realationship needs to be established as to minimum sizes of particles and imperfections that can be tolerated with respect to light source distance from the exposure plane.
Please include all calculations and assumptions to that we may follow the methodology for your conclusions. I would like to be able to validate this work.
Good lord Alex, I think you are on to something here.The paper to film thickness variation was obviously the key to any significant variation that the printing frame latching system might possibly yield. We were damn fools not to see it before. However is the notion of the X and Y axis still valid when we still have no definite proof that the minimum size and tolerance ratio of glass imperfection has still not been at least 97% validated.
The answer awaits.
Count me out of this thread. I enjoy printing too much to look any more!
Cheers and good luck,
Mike, the pressure variation in both the x and y directions is important so that a pressure compensating mechanism can be constructed. One of the major problems with traditional latches is that the pressure tends to concentrate where the latch is attached or makes contact with the pressure back. It then deceases radially until it is met by the similar pressure gradient from an adjacent latch.
Originally Posted by Mike Crawford
This needs to be known before the defect sizes are determined since the minimum allowable defect size will vary inversely with the pressure gradient. With current designs, different size particles and glass defects have varying degrees of effect on the prints.