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Thread: Surface & Internal Image theory

  1. #1

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    Surface & Internal Image theory

    Anyone else familiar with a book by W.F. Berg, Exposure Theory & Practice. I'm reading through it and have come across a section on RF where he gets into the technical aspects of its effects on film grain. Part of the discussion is about the surface image and internal image on the film grains, this is a concept I was not aware of, can someone simply explain the surface and internal images, what they are and how they work?

    This discussion on RF is addressed in terms of low intensity and high intensity exposures, and how both are handled by fast and slow speed films, as well as b&w, c-41 and e-6 films. Without going deep into the specifics the chemical response of film grain to long and short exposures as described in the book, there is a reaction that creates a surface image and an internal image. If I have grasped this correctly the surface image disappears early in development and the internal image is set in the next phase of chemical reaction.

    While the book is quite technical and stretches my thinking, I'm enjoying the challenge of going into uncharted and possibly superfluous areas of photographic understanding, particularly around exposure and development concepts.



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    Nodda Duma's Avatar
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    Re: Surface & Internal Image theory

    Internal and external image refers to the formation of the latent image speck upon exposure either internal to the silver halide grain or on the surface.


    https://en.m.wikipedia.org/wiki/Latent_image
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    Re: Surface & Internal Image theory

    Thanks ND, that makes sense of it. Now I'm interested in testing the different developing methods and conditions and their influence on the latent image being processed. Most of my work is low intensity, this helps understand the chemical reaction in the process of developing.

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    Steven Ruttenberg's Avatar
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    Re: Surface & Internal Image theory

    This sounds interesting being I am an engineer with a physics background.

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    Re: Surface & Internal Image theory

    Jason pointed here https://en.m.wikipedia.org/wiki/Latent_image information about position o the latent image inside an emulsion...

    But let me speak about another thing, how a BW film may have two separate emulsion layers, this impacts in the grain structure we perceive.


    A film like TMX is able to resolve 63 lines/mm in 1.6:1 contrast conditions but 200 lines/mm with 1:1000 contrast (datasheet). This is from 31.5 lp/mm to 100 lp/mm, because a line pair (lp/mm) is 2 lines.

    TMX has two emulsions inside, in two separated layers. The outer layer facing the lens it's the high speed layer that have larger sensitive crystals, this layer is panchromatic, with that particular spectral response that governs TMX tonality from colors in the scene translated to gray. Of course this layer is formulated with a mix of different crystal sizes and sensitized to obtain the amazing performance that kodak (and Sexton) liked. In this emulsion we have a lot of flat chrystals, as it is of Tabular T-Grain type. IIRC (from Making Kodak Film book) we can obtain flat grains by ajusting pAg concentration during crystal growth.


    But that amazing outer emulsion is not perfect, it can be burned in the highlights at some amount of exposure. So to extend the recording of detail at higher exposures, and the linearity of the sensitometric curve in the highlights they place another different emulsion under the main emulsion, the low speed emulsion.

    That low speed emulsion has a quite slow ISO, so it builts additional density with increasing exposure when the (main) outer emulsion is saturated and not recording more detail in the highlights, the capability to record highlight detail it is even increased because the outer emulsion casts a shadow on the inner one, working a bit like if it was a ND filter for the inner emulsion.

    That inner slow emulsion is (IIRC) orthochrmatic (red insensitive) and of classic cubic grain, with very small grains. Those small grains, when exposed, are able to produce a high resolving power, I guess that TMX produces 200 lines/mm resolving performance at 1:1000 contrast because in those conditions is when we see the inner emulsion in action, 1:1000 is 10 stops dynamic range.

    The capabilities of film when recording highlight detail are amazing, it's hard to belive the amount of extreme highlight detail we can recover from it. This comes from the formulation (grain size diversity) of emulsions and from layered emulsions.

    Digital devices have been trying to get those capabilities. The antique Foveon sensor had pixels of different sizes, and Arri Alexa movie cameras have DGA, dual gain amplifier technology, the same analog readings of the pixels are feed into two separated amplifiers working at different ISO...

    The highlight detail is critical in Hollywood, because glares in the faces of actors are very important to depict face volumes and skin textures, you always pay too much for a charming actress if at the end you are to depict her face volumes as it was the face of a president Reagan on a dollar coin. Note that digital camera manufacturers always speak about high ISO and never about highlight detail, when lack of light is not a problem in Hollywood, they have truckloads of lightning equipment. This 2018 the challenge in a good movie still it is highlights, not the shadows.

    This is a factor contributing to the present situation, film is testimonial in the industry but this 2018 still some selected movies were still shot in film, for example Mission Impossible: Fallout or StarWars VIII. Anyway digital cameras are always improving...

    So this is the importance on the inner emulsion and/or grain formulations, IMHO.


    Regarding aethetic grain structure (mostly important in smaller formats), each level of density in a film has a particular coarseness, some films have more grain in the mids and some more in the shadows.

    TX/TXP has more grain in the shadows than in the mids, while HP5 has more grain in the mids than in the shadows. From that, depending on how we expose a film, we'll have more grain in an area or in the other.



    https://www.flickr.com/photos/125592...5/28286548926/

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    Re: Surface & Internal Image theory

    Quote Originally Posted by Pere Casals View Post
    That inner slow emulsion is (IIRC) orthochrmatic (red insensitive) and of classic cubic grain, with very small grains. Those small grains, when exposed, are able to produce a high resolving power, I guess that TMX produces 200 lines/mm resolving performance at 1:1000 contrast because in those conditions is when we see the inner emulsion in action, 1:1000 is 10 stops dynamic range.

    ...

    some films have more grain in the mids and some more in the shadows.

    TX/TXP has more grain in the shadows than in the mids, while HP5 has more grain in the mids than in the shadows. From that, depending on how we expose a film, we'll have more grain in an area or in the other.
    I think you are confusing the structure of TMX & TMY-II slightly here. And neither have Ortho sensitive only layers - it would look extremely weird at the crossover between the emulsions - I think you are confusing absorber dyes with sensitising dyes. Ilford use similar technology in the Delta films in terms of flat crystals & cubic crystals in different parts of the emulsion structures. Essentially you're using different kinds of monodisperse crystals to try & get the benefits of polydisperse crystals without the downsides. TMX may be blended emulsions rather than multilayer, though I'm not sure. It was available as a glass plate product & it's never been entirely clear to me whether there was ever much work done on multilayer coating on plates - especially given that the majority of the glass plate market in the later 20th Century was BW emulsions for specific scientific purposes. It is worth noting that Agfa's documentation shows that APX25 & APX100 had single emulsion layers & APX400 had two. Given the relatively limited ways to skin the cat that is emulsion making, it's likely that their competition were/ are not dissimilar in approach.

    Furthermore, TMX is unusually accurate in terms of colour representation, such that it is recommended as a separation film.

    Finally, regarding 'more grain in the shadows or mids', that has more to do with printing approaches, exposures, processing time & how expired your film is (sorry, but 12-years out of date TX is not a valid comparator) than perceived inherent aspects of the film. It isn't difficult to create prints that would fool someone's received wisdom of what TX & HP5 'are', but if you understand the actual characteristic curves & how they behave relative to process time etc, telling them apart becomes relatively easy.

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    Re: Surface & Internal Image theory

    Quote Originally Posted by interneg View Post
    TMX may be blended emulsions rather than multilayer
    quoting: "T-Max has a combination at the top of the emulsion are two layers of 'tabular' grains that are larger at the top record and smaller in the lower one they are uniform in size (monodisperse).
    Beneath that is a layer of 3D grain (Kodak speak for cubic type) that is finer and slower probably polydisperse."

    For simplicity I didn't mention that the T outer layer is in fact composed of two layers...




    Quote Originally Posted by interneg View Post
    Furthermore, TMX is unusually accurate in terms of colour representation, such that it is recommended as a separation film.
    Well, the spectral sensitivity curve does not show a flat spectral sensitivity... anyway TMX is suitable for color separation with the right illumination and calibration.

    Click image for larger version. 

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    Quote Originally Posted by interneg View Post
    Finally, regarding 'more grain in the shadows or mids', that has more to do with printing approaches, exposures, processing time & how expired your film is (sorry, but 12-years out of date TX is not a valid comparator) than perceived inherent aspects of the film. It isn't difficult to create prints that would fool someone's received wisdom of what TX & HP5 'are', but if you understand the actual characteristic curves & how they behave relative to process time etc, telling them apart becomes relatively easy.
    Please read comments in the flickr link, that sample does not show actual film images but how a film grain simulation tool uses different (scientifically measured) curves for density vs grain_size. Just to show the shape of that curve.

    For real samples you can see this TX image, you may note that on the car the darker areas have more grain, see similar HP5 shots and you'll see that peak graininess is more in the mids, anyway this depends on exposure.




    Quote Originally Posted by interneg View Post
    And neither have Ortho sensitive only layers - it would look extremely weird at the crossover between the emulsions
    No... because the possible ortho layer is of extreme low ISO and works only with extreme highlights, showing textures of glares and illumination sources, that are usually neutral in color. I'm not completely sure that the cubic (very low ISO) deep layer is ortho, IIRC it is. A factor to not sensitizing it panchro would be a simpler process, and not increasing emulsion speed, as that emulsion is responsible for the linearity in the extreme highligts, where TMX/Y is extremly linear compared to other films that have more or less a shoulder.

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    Nodda Duma's Avatar
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    Surface & Internal Image theory

    Back to internal vs external, I think it’s Tadaaki Tani who goes into the specific details of the differences and impact on emulsion sensitivity between the two in his book “Photographic Sensitivity: Theory and Mechanisms”.

    In short, emulsions with external or just-under-the surface developing centers are practically more sensitive since the developer can get to the latent silver atoms easier (either directly at the surface, or slightly dissolving the halide crystal if just under).
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    Resident Heretic Bruce Watson's Avatar
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    Re: Surface & Internal Image theory

    Quote Originally Posted by Colin D View Post
    Anyone else familiar with a book by W.F. Berg, Exposure Theory & Practice.
    ...
    While the book is quite technical and stretches my thinking, I'm enjoying the challenge of going into uncharted and possibly superfluous areas of photographic understanding, particularly around exposure and development concepts.
    If you like that one, you should read (that would take awhile as both volumes are absolutely huge) Grant Haist's Modern Photographic Processing. Detail you won't likely find anywhere else, all backed up by his career as a lead research chemist for Kodak. The man knew a bit about photo-chemistry.

    Hard to find a copy to own; your best bet may be a university research library. At least the university closest to me has a copy.

    Bruce Watson

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