How does this work?

Discussion in 'B&W: Film, Paper, Chemistry' started by tiberiustibz, Mar 30, 2009.

  1. tiberiustibz

    tiberiustibz Member

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  2. smieglitz

    smieglitz Subscriber

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    My guess is the paper is pre-exposed at the factory to the brink of solarization. Then, any additional camera exposure would produce a reversal of tones giving a positive image.

    Joe
     
  3. Photo Engineer

    Photo Engineer Subscriber

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    Actually there are 3 ways to get direct reversal.

    One is as you describe Joe, but there are 2 others. One involves the normal B&W reversal process and the other involves a direct reversal emulsion. This is typically a core shell emulsion which gives a positive image upon exposure. I'm sure you don't want chemical details but they are well known in the literature and in patents galore. There are basically 3 types of direct reversal emulsions, Reversal F, Reversal P and hmmm, I foret the formal name of the 3rd type OTOMH. Sorry. But, anyhow, Reversal F was used in Kodak's PR-10 instant film and also a color product called Directochrome which was only marketed in Europe for a short time.

    Nice stuff when you can get it.

    PE
     
  4. wogster

    wogster Member

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    So how do these direct reversal emulsions work?
     
  5. Photo Engineer

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    Well Paul, I assumed you did not want the chemical details. :sad: Never make assumptions.

    Ok, the center of the grain is fogged to dmax and the surface has what is called a nucleating agent on it (or it can be in the developer) and this causes the latent image sites on the surface to essentially vanish and then the core develops, so you get a reversal image or a direct positive image.

    To achieve a fogged center, you usually make a core and fog it, then build a normal emulsion over the foggy core and you then sensitize it normally. Nucleating agents differentiate the different types (R and P) and the "R" type are generally hydrazides. I forget the details of the others, as the "F" is what I worked with mostly.

    It requires rather coarse grains and achieves rather slow speeds. It is also subject to re-reversal giving both negative and positive images like the positive materials version of solarization. Bright objects have a black dot in the center in those cases, from the combined neg-pos image. They are not perfect, but were used for several Kodak products. They are no longer in general use. The highest speed I ever heard of was in the range of about ISO 50 - 100 and the grains were huge. They were never used in films AFAIK due to the huge grain size.

    PE
     
  6. mjs

    mjs Member

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    Sounds great for 4x5 pinhole. Hmm, I happen to have one of those (thanks again, Joe!) and am getting ready to place an order next week... :wink:

    Mike
     
  7. nworth

    nworth Subscriber

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    PE's comments sound something like the old Kodak Autopositive reprographic film and paper. It was extreme contrast, but I see no reason why more normal contrast wouldn't be possible. The solarization type emulsions usually have a lot of base fog. I remember that Autopositive had none, but that you had to expose it through a yellow filter or it would be all fog.
     
  8. tiberiustibz

    tiberiustibz Member

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    My Fujiroid film has the black dot syndrome too.
     
  9. Photo Engineer

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    Well, AFAIK, it does not use direct reversal emulsions. So there! :D

    PE
     
  10. alanrockwood

    alanrockwood Member

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    PE, could you explain this part in more detail? I don't quite follow it yet.

    Thanks.

    Alan
     
  11. Photo Engineer

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    Most simply put, the nucleating agent inhibits the surface negative image and internal image as well, but where there is no exposure, it allows the internal fog to develop as if it were image. Thus, no negative image forms and only a positive "fog" image forms.

    PE
     
  12. Joe VanCleave

    Joe VanCleave Member

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    Here are two threads over on F295 about my experience with using this Efke direct positive paper.

    Overall I find it needs significant preflash, more than I normally give paper negatives, to get anywhere near normal contrast, even when rating the EI at less than 1.

    ~Joe
     
  13. Photo Engineer

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    Joe;

    I would imagine that this type of paper would suffer from a huge reciprocity effect. With a pinhole camera, you might be seeing that due to the long exposures. IDK for sure though. Any thoughts?

    PE
     
  14. Joe VanCleave

    Joe VanCleave Member

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    PE:

    You're probably correct about the huge reciprocity effect, but this link refers to me using an exposure of ~1.5 - 2 seconds at F/16 in an improvised refractive lens, LF box camera, for the several self-portraits posted. I'm not sure if 1-2 seconds implies significant reciprocity or not.

    I'm also curious about using elevated developer temperatures with this paper; tests I did this week of Arista's grade 2 RC (conventional) paper, used as an in-camera negative in both glass and pinhole lensed cameras, indicates I can get nearly EI=25 by using Ilford's Universal paper developer, diluted 1+15, at 80f. That post can be found here.

    None of my tests are textbook scientific in accuracy, however; they merely represent the degree of accuracy I can get using my normal processing method.

    ~Joe

    PS: My avatar is from one of the Efke direct positive self-portraits.
     
  15. Photo Engineer

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    I would tend to think that 1 - 2 seconds would be too short for a huge amount of reciprocity to take place, even in this type of paper. I do know that these emulsions can be made to yield a variety of contrast ranges depending on sensization and silver levels.

    I did notice your avatar. :smile: If I had one, it would be yellow with a red "K" on it. :smile:

    PE