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...
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.
Wow Ian (Hexavalent), you must be getting good if you're being mistaken for PE! :laugh:
Originally Posted by glbeas
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.
Too Funny! Now none of my hats fit ;)
Originally Posted by holmburgers
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...:-)
The whole point of the exercise, though, was to be able to make continuous-tone pictures using semiconductor photoresist, because it's safe to say that photoresist is going to be around longer than litho film. I guess I need to find a photoresist that is sensitive to near-visible light or the deal's off anyway due to the UV thing.
I think you should still try it. It's just a matter of how long the exposure takes, not if it will take. It will, it just might take a day or two.. haha, hopefully not.
Picked up a Polaroid half-tone line screen (85 lines per inch) on eBay recently. It's made for some Polaroid camera, but it's 4x5" and should work well for that format as well.
The trick will be mounting it and adjusting the spacing.
However, what does anyone know about making analog half-tones? Like approximately how far from the film should the screen go? My limited understanding is that distance from the film and the f-stop allows for control over the half-toning.
Does anyone know of old trade publications, or instructional pamphlets for making half-tones? Surely there's a lot, this just isn't a field of research I've really looked into.
This is a Kodak Gray Negative Contact Screen. It is an 85 line eliptical dot screen. It is 8x10 inches. The Kodak stock number is 148 6399.
Can anyone tell me what exactly this is for? Is it for creating half-tones by contact printing, or something else?
It is for creating halftones from continuous tone subject. It is placed in contact with the film (or photosensitive material) as a sandwich using a weak vacuum to maintain contact. You could use it for contacting a negative, or expose in a process camera. It would be "hard" to use in a regular camera because it would be hard to make a sandwich with the screen in close contact with the film.
85 lines is relatively coarse, newspaper grade, suitable for a hand-coated high-contrast light sensitive material. Great for experimenting because it will give successful (at least interesting) results even if your material is crude or the contact is imperfect.