In-camera half-toning

Discussion in 'Alternative Processes' started by BetterSense, Jan 7, 2011.

  1. BetterSense

    BetterSense Member

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    I think it's safe to say that I don't understand half-tone processes very well, but I'm pretty sure that they are designed to give something that looks like continuous tone results when working with essentially infinite contrast media like lithographic plates.

    Suppose the only film in the world was litho film or similar "all-or-nothing" media. Is there any way to use a halftone screen in the camera itself so that you could achieve pictorial results?
     
  2. Bill Burk

    Bill Burk Subscriber

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    Hi BetterSense,

    If you have a process camera with a vacuum back, it could be used for pictorial results. I did a self portrait once.

    The halftone process does exactly what you explain. It breaks a picture into a grid. You'll print tiny dots for highlights, checkerboards for middle gray and almost solid with tiny white dots for shadows. To make a halftone negative you would use a screen to project a fuzzy grid on the high-contrast film. The very earliest screens were two sheets of thin glass with parallel lines etched and inked. Then the two sheets of glass were crossed at 90 degrees back to back to make a checkerboard pattern. Held at a specific distance from the film, each checkerboard square projected a fuzzy image. Where the subject was bright, the whole square would go black but where it was dark the square would only get a threshold pinpoint of dark on the negative. I never actually saw one of the glass screens except in old books.

    Later there were contact screens which were basically continuous tone copies of the shadow that the glass screens would project on the film. This is what you probably could get your hands on. You would lay it on top of your film and use a vacuum easel to get the air out.
     
  3. photoncatcher

    photoncatcher Member

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    My God. This thread brings back some interesting memories. I spent almost 8 years as an offset camera operator. Not only did I shoot thousands of B&W halftone negatives from B&W prints, and drawings, but I also did 4 color seperations on a process camera. The camera took up two rooms (one was a dark room) had a copy board that was 40x50 inches, and a film back that was 30x40 inches. It also had four dual tube pulsed xeon lights to illuminate the art work. A single color seperation could take up to 3 to 4 hours.
     
  4. htolino

    htolino Member

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    Polaroid used to sell a halftone kit for their pack film cameras and backs. I used to have one for my MP-4 back in the 80s; sadly, I don't recall ever using the beast.
     
  5. BetterSense

    BetterSense Member

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    So If I wanted to do something like this with a 4x5 camera, how would I go about getting a halftone screen and how far would I put it from the film?
     
  6. Hexavalent

    Hexavalent Subscriber

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    A print shop may have half-tone screens leftover from the pre scanner/digital days.

    Screens are exposed in contact with the film - there is also the issue of using flash and bump exposures to obtain a full tonal scale, but that is an entire world of alchemy on its own :smile:
     
  7. richard ide

    richard ide Subscriber

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    You could only shoot halftones in a camera with a vacuum back. Any air space between the film and the halftone screen film will give you a fuzzy image in that spot. When I made halftone exposures in a process camera; in addition to vacuum, I used a roller to remove air pockets before exposures.
     
  8. Mustafa Umut Sarac

    Mustafa Umut Sarac Member

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    If you are serious going with this , I advise you to use rotogravure elliptical screening like National Geographic Magazine uses. I think you can go from more ordinary way and get your scan file and send to prepress shop via email and if you want a black and white , order the cyan rotogravure screened film.
    If they have automatic printer with a developer , it takes their 5 minutes.
    At 1994 , AGFA presented crystal screening and it was made using smallest screening dots and density of them. I am far from the sector for 10 years but I think Heidelberg , AGFA , Kodak , Fuji have screen printers. Visit them , learn their technology , select one , find a prepress office who does it and voila.
     
  9. BetterSense

    BetterSense Member

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    I originally asked this because I wanted to try photography using glass plates coated with photoresist. I work in the semiconductor industry and regardless of what happens to film, we will always have semiconductor photoresist as long as electronics are being manufactured. The problems with using photoresist are that it's an infinite-contrast medium like litho film, and it's only sensitive to UV light.
     
  10. Bill Burk

    Bill Burk Subscriber

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    Hi BetterSense,

    Now you're talking. The "contact halftone screen" needs a vacuum as already pointed out. But older technology glass screen will solve that problem because you have to shim it a certain distance away from the lith film to get a penumbra.

    Get the photoresist guys to make you a linescreen grid on glass. A checkerboard pattern. They have the tools to draw lines an atom thick. They could make a checkerboard right off the bat (the old timers had to draw straight lines and cross them for a sandwich. For your purpose look for maybe 65-85 lines per inch. You can experiment with available lith film. There may be unevenness due to your film not lying flat.

    Mustafa,

    I always appreciate your ideas to apply technology in unexpected ways
     
  11. BetterSense

    BetterSense Member

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    I wasn't talking about making the halftone screen using photoresist. I was talking about using photoresist as my pictorial imaging negative--in place of film. The idea would be to spin-coat photoresist onto a glass plate and then use that in-camera, develop it, and contact or enlarge it.

    Since photoresist is not a continuous-tone medium, the only way that would work would be using halftone techniques.
     
  12. Bill Burk

    Bill Burk Subscriber

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    I get that. You will need a halftone screen.

    You can buy a "contact halftone screen". Check auction site completed listings. It's like a sheet of film with a negative of a bunch of fuzzy dots. For that you will need a vacuum back.

    I've never seen a glass halftone screen on an auction site, but I know they used to exist. This is what I think you can get your photolith department to conjure up. It won't be light sensitive, it will be a checkerboard.

    You'll hold this glass checkerboard a very slight distance away from your sensitive plate. Just enough that the shadows don't overlap, but make a continuous pattern of dark to light. As each dot under the screen opening gets hit by more light, first a small dot forms, then it grows as more light hits it.

    For now, just hold a windowscreen over a sheet of paper, you'll be able to see the effect.
     
  13. holmburgers

    holmburgers Member

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    I'm interested in doing something similar and have just beeen searching the archives, only to find this pretty-darn-recent post.

    What I want to do is transfer a carbon-gelatin image onto copper (or other metal), and then use a patina formula to get a 'patinograph'. Again though, one would need a half-tone negative to get gradations.

    There appears to be some halftone screens on eBay. They are all magenta in color, and say 'magenta'. Why is this? Do they have cyan and yellow? I don't quite get why the screen itself has to be colored, as it itself is presumably not a separation filter. Or is it?

    BetterSense, have you attempted anything thus far?
     
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  15. Steve Smith

    Steve Smith Subscriber

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    If you use copper clad PCB material with a photo resist and etch it in ferric chloride, you basically have an etched copper plate which you can use to print with. Ink it up with a roller and press it onto paper (I don't know what a patinograph is but I'm going off to find out now!).


    EDIT:

    I failed!!! This is what Mr Google said:


    Steve.
     
  16. holmburgers

    holmburgers Member

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  17. andrew.vartabedian

    andrew.vartabedian Member

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    I'm sure this already crossed someone's mind, but it sounds like the solarplate process is similar and could hold a solution. In it, one exposes the plate using a halftone screen and then exposes the plate to the image. Perhaps one could use a random-dot half tone (aquatint) screens to expose the photoresist plate, then remove the screen and expose the plate to the image (either through contact printing or through a camera). Depending on the nature of the photoresist plate, the half-tone prep could be done well in advance of the image exposure.
     
  18. holmburgers

    holmburgers Member

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    Here's a picture of one of the screens... http://www.imcclains.com/catalog/blocks/aquatintscreens.html

    Sounds promising.

    I don't quite understand the double-exposure; I thought that usually they were exposed simultaneously, but even this seems fuzzy to me, as in, how does that create half-tone from continuous tone?
     
  19. glbeas

    glbeas Member

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    The film used with the screen is blue/green sensitive so the red light that gets through a magenta screen has no effect on the image. I think the magenta was used for more speed in the exposure. I've used both magenta and grey screens and saw no real difference in how well they worked. One thing to consider is to look for an elliptical dot screen, these produce smoother gradations in the mid tones than a round dot screen.
     
  20. holmburgers

    holmburgers Member

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    Thanks Gary, very good to know.
     
  21. Hexavalent

    Hexavalent Subscriber

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    "Aquatint" screens are a replacement for classic technique of using rosin powder in a "dust box" with copperplate etching. Tones are reproduced by means of the ink held in the etched areas - depth and size of the etch plus the viscosity of the ink used are the major determinants of the tonal range produced.

    Think of the double exposure somewhat akin to getting the image 'off the toe'.
     
  22. BetterSense

    BetterSense Member

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    The pre-exposure idea sounds like the best one yet. It sounds promising because it should require no modification to the camera. I haven't tried any of these methods yet because I have to get my hands on some photoresist and some kind of plate holder, and some kind of halftone screen...
     
  23. glbeas

    glbeas Member

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    The pre exposure or "bump" exposure like PE says gets the film off the toe by exposing it to the point of making a very tiny dot in the unexposed area. Any further exposure creates bigger dots. Without this the shadows would be totally empty of dots. This was normally done with a safelight with a yellow filter in it hanging over the vacuum back with an enlarger timer running it. You calibrate this to your process by experimentally determining the smallest dot that will make a discernable dot on the finished image, in my case it was on the newsprint after the press printed it from the plate made with the halftone. How the image looks at those in between stages can be a bit odd to those not used to working with dot gain and slur.
    On the other end of the greyscale the highlights should be left with a white dot on the neg just big enough to create the tiny specks in the hightlight areas, mainly controlled by the main exposure and fine tuned by development inspection.
    Remember too that not all lithographic films are the same "infinite" contrast. Cheaper line films had lower contrast and would make a fuzzy edge dot that is hard to calibrate and make good positives from. There were films made especially for doing halftones with and worked best with the respective brand of developer it was made for but usually did quite well with whatever litho developer was on hand.
    There were also a few films made that would make halftones without a screen. I never had my hands on any of that so I can't tell you much about them and I doubt you would be able to find any very easily that was still good.
     
  24. holmburgers

    holmburgers Member

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    Wow Ian (Hexavalent), you must be getting good if you're being mistaken for PE! :laugh:

    That makes sense though.

    BetterSense, I think that your in-camera exposures onto a photo-resist plate would be enormously long unless you plan to use a plastic lens. Glass doesn't pass UV well, let alone w/ multi-element lenses. Oh, and omit the UV filter... haha.
     
  25. Hexavalent

    Hexavalent Subscriber

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    Too Funny! Now none of my hats fit :wink:
     
  26. glbeas

    glbeas Member

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    Hah, sorry about the confusion. I think the job would be better done with lithographic film, then contact printed onto the resist. Thats the way the system was designed in the first place and will be easier and more efficient.
    Ian, pull that hat a little tighter...:smile: