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  1. #1

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    Grains - what is it, really?

    I am trying to understand the role of grains on film and what it really means to me in terms of image creation. I understand as much that grains on film are metal silver that developed from silver halide.

    Is a grain the smallest element of image? Saying this in different way, must an entire piece of grain must turn into certain shade of gray or can a grain turn into different shade in different parts? Is the size of grain the limiting factor on resolution of the image?

    When are the grains created? At the time emulsion is placed on the sub-strait or at the time developer turns latent image into a real image? I thought the grain may equal to the crystal that were created when silver halide is placed on film but then if so, the size won't be different when different developers are used. Some developers are known to create smaller or larger grain.

    Can someone explain please, or point me to a material that may explain this?

    Thank you.

  2. #2
    keithwms's Avatar
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    It's actually quite a long story. Let me suggest that you first look at the wikipedia article on the latent image.

    You might also look at slide 35 in this colloquium I gave:

    http://www.phys.virginia.edu/Announc...ides/S1433.pdf

    After that, you might be inclined to look at the broader reviews on photographic sensitivity, which explain it all in more detail. Do bear in mind that not all b&w film has the same grain structure, not all has the same sensitization mechanism, and not all of it is developed the same way.
    "Only dead fish follow the stream"

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  3. #3

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    Thank you, thank you, thank you. I think, page 35/48 of the pdf above illustrates what I was really asking. Page 71 almost caused me to spit out my drink from nose....

    So, what I understand from all this is, that GRAIN is the smallest unit in image forming then. The photons from incoming light turns the crystal "on" and the development process turns the "on" crystal into silver which we recognize as 'grains'.

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    Quote Originally Posted by tkamiya View Post
    So, what I understand from all this is, that GRAIN is the smallest unit in image forming then.
    Almost but not quite. I think it would be more correct to say that the sensitivity centers are the smallest unit in the image formation process. But in the final image, yes, the grain is arguably the smallest unit. But again, there are several different development processes and for some you see dye clouds as the smallest unit of the final image.

    Perhaps Ron or one of the other experts can clarify further....
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    Be careful about this, because the "grain" we see, tends to actually consist of a clump of individual grains, each of which is smaller than what we are able to see.

    Matt

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    Can you give me an idea of what kind of scale we are discussing here? At x1000 magnification (under microscope) if I see a particle, am I looking at individual grains or a clump?

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    keithwms's Avatar
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    That's true, what we see is not the same as what is actually the smallest information-carrying unit. The grains are measured in microns; the smallest 'dots' we discern are a good bit larger. But there isn't one answer for all films, nor even just b&w films- there are many different kinds of grain, and if you develop with stains then you get a different visual effect. Some have a distribution of sizes and some are more uniform. One of the slides in my talk shows some different kinds of grains as viewed by electron microscope.
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    So, in realistic sense then, a grain AS WE SEE IT, can have more than one shade because it contains hundreds of actual crystals/grains and each can be in different shade. I would imagine, since we are talking about clumps, they are not distributed evenly. I also see, on page 55, distribution and the size of grains aren't even either.

    At 10^-6, it is at the edge of resolving power of my microscopes....

  9. #9
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    I thought that the 'grain' we see on the print is actually the hole between the grains in the film (negative). I thought that the grain in the film is somewhat 'stopping' the light to travel trough the film emulsion and to hit the photographic paper. So, if this is right, then what we see on (in?) the print is a kind of a reversed image of the film's grain in the sense that it is the supposed 'surrounding' of the film's grain.
    To say it with a little boutade, the grain in film is slain by light on to the paper (where in Spain it is slain by the rain)...
    Or am I wrong again?

    Philippe
    "...If you can not stand the rustle of the leafs, then do not go in to the woods..."
    (freely translated quote by Guido Gezelle)

    PS: English is only my third language, please do forgive me my sloppy grammar...

  10. #10
    AgX
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    Quote Originally Posted by MattKing View Post
    Be careful about this, because the "grain" we see, tends to actually consist of a clump of individual grains, each of which is smaller than what we are able to see.

    Matt
    Yes, we have the grain that is just the crystal, then we have the grain that is the developed crystal, an aggregate of silver in different shapes, depending on processing, and last the optical impression all the stacked aggregates of silver make on the viewer/the print when illuminated from behind.

    The latter is the actual `element´ of image forming. Or their equivalent of dye-clouds.


    But to make things even more complicated, on the level of silver aggregates, the way of illumination is of influence too (Callier Effect).

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