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  1. #1
    tiberiustibz's Avatar
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    How does this work?

    http://www.freestylephoto.biz/22242-...-4x5-25-sheets

    How? This boggles my limited photography mind...

  2. #2
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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. #3
    Photo Engineer's Avatar
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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. #4
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    Quote Originally Posted by Photo Engineer View Post
    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
    So how do these direct reversal emulsions work?
    Paul Schmidt
    See my Blog at http://clickandspin.blogspot.com

    The greatest advance in photography in the last 100 years is not digital, it's odourless stop bath....

  5. #5
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    Well Paul, I assumed you did not want the chemical details. 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. #6
    mjs
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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...

    Mike

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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. #8
    tiberiustibz's Avatar
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    Quote Originally Posted by Photo Engineer View Post
    Well Paul, I assumed you did not want the chemical details. 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
    My Fujiroid film has the black dot syndrome too.

  9. #9
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    Well, AFAIK, it does not use direct reversal emulsions. So there!

    PE

  10. #10

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    Quote Originally Posted by Photo Engineer View Post

    ... 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...

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

    Thanks.

    Alan

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