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What is the physical difference between fast and slow stocks?


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#1 grantsmith

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Posted 05 November 2006 - 10:23 AM

hi,

What is the physical difference between fast and slow stocks?

Is it the size of the silver particles? or is it the amount? or something else?

I know slower stocks are referred to as having finer grain but I'd like to know the physics behind it.

thanks
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#2 John Pytlak RIP

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Posted 05 November 2006 - 05:03 PM

hi,

What is the physical difference between fast and slow stocks?

Is it the size of the silver particles? or is it the amount? or something else?

I know slower stocks are referred to as having finer grain but I'd like to know the physics behind it.

thanks


Mostly it is the size and shape of the silver halide grains. Kodak's invention of "T-Grain" technology in the early 1980's was a significant breakthrough in reducing graininess and improving sharpness. Likewise, Kodak's introduction of DIR couplers in the early 1970s. Here is a good tutorial:

http://www.kodak.com...html#graininess

And some of the technology in the Kodak VISION2 color negative films:

http://www.kodak.com...s/vision2.shtml
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#3 grantsmith

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Posted 05 November 2006 - 05:07 PM

thats great. thanks john. hope everythings going well with you
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#4 Dominic Case

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Posted 05 November 2006 - 08:51 PM

A silver halide crystal needs to be struck by three or four photons to become "exposed' so that it will be developed into a part of the image.

However big or small the crystal is, it still only needs the same number of photon strikes.

If a certain area of the emulsion surface is lit by a certain intensity of light - that is, a certain number of photons per unit area strike it during the exposure time - it's clear that a large crystal is more likely to score the required hits than a small crystal. Or even a number of small crystals covering the same area as one large one.

So it follows that emulsions with large halide crystals are more sensitive to light (i.e. "faster") than ones with smaller crystals (more "fine grained").

Emulsions usually have crystals of a range of sizes, which extends the dynamic range (large crystals capture shadow detail, smaller crystals kick in in the highlight area as well as filling in some of the detail in the shadows.

T-grain technology essentially creates flat crystals and has them lying flat in the emulsion coating so that they present their largest face to the light, maximising their sensitivity for the same size of crystal.
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