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Thread: side by side comparison... large print digital back VS 4x5 color film

  1. #41

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    Re: side by side comparison... large print digital back VS 4x5 color film

    Quote Originally Posted by Oren Grad View Post
    This makes no sense in purely mathematical terms. By definition, the expected value will never be the smallest grain size unless the grain is uniform in size. And in that case, talking about the "average grain size" is superfluous.
    A little clumsy wording on my part. Edit "smallest" to "smaller" and it makes perfect sense to me.

    I am trying to say one sets the aperture to match the average size of the grain. I was trying to exclude any large grain clumps from being included in the average.

    Thanks,

    Tim

  2. #42
    Resident Heretic Bruce Watson's Avatar
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    Re: side by side comparison... large print digital back VS 4x5 color film

    Quote Originally Posted by Tim Povlick View Post
    While I would agree film grain clump size can be viewed as a Stochastic function, the is not the reason why. Can you elaborate on this? I checked "Probability, Random Variables, and Stochastic Processes" by Papoulis.
    You're not likely to find it in a statistics text. This is about how film works. A stochastic process implies a level of randomness with an overlay of a guiding element. In this case the randomness is the distribution of silver halides in the emulsion and the guiding element is the exposure of image itself.

    I'm just talking about metallic silver grain here. In color films dye couplers are used to replace the metallic silver grain clumps with dye clouds. So where I reference grain clumps you can substitute dye clouds and get the same basic meaning for color films.

    Areas of low exposure (shadows) tend to capture the few photons that come their way at the surface levels of the emulsion. Areas of high exposure tend to capture photons throughout the depth of the emulsion. After processing, the resulting image show less graininess in the areas of low density and considerably more graininess in areas of higher density.

    So far so good, yes? What makes graininess interesting is how it's formed. It's not just individual film grains. It's groups of grains, usually interlocking strands of metallic silver. These groupings form both in 2D and in 3D. That is, in the direction of the plane of the surface of the film, and in the depth of the emulsion. These groupings are commonly referred to as grain clumps. Note that overlapping of grain clumps as you look into the depth of the emulsion just makes them bigger. This is the view the scanner uses.

    Finally grain clumps work together to create density in the film. There are at least two ways this happens. First, the size of the grain clumps increases. Second, the interstitial spaces between the grain clumps decreases. The net effect is that it's more difficult for light to pass through.

    So... as grain clumps grow and become closer together they create the film's density. I believe I've read somewhere over the years that depending on the film, processing, and the image, that film grain clump size can vary a couple of orders of magnitude. Say from 1 to 100 microns.

    To say that you can make any kind of a match between a single aperture and this range of grain clump sizes is, to me at least, quite a stretch.

    Quote Originally Posted by Tim Povlick View Post
    Back to the main point, scanning and film grain, (or whatever one wants to call it), the aperture is being set not to match simply the grain size of film, but the average size of the smallest grain.
    Huh? The average of the smallest grain? I don't know what the means.

    Quote Originally Posted by Tim Povlick View Post
    In Stochastic terms this would be the expected value of grain size.
    I don't follow you. You could use a statistical function to get an average or the mean maybe. Are you implying the RMS average grain size?

    Quote Originally Posted by Tim Povlick View Post
    So I agree with what Aztek says, as long as one realizes it's the average grain size that set's aperture.
    I would at least not argue so much if that's what they said. But as I recall they leave out that pesky word "average."

    Quote Originally Posted by Tim Povlick View Post
    Therefore one should not vary aperture while scanning,
    Should not? How about can not? I've never seen a scanner that would vary aperture during a scan. I wonder what kind of results you'd get if it did. Hmmm...

    Quote Originally Posted by Tim Povlick View Post
    If one is set to 10 microns and along comes a dark area that is "clumped" at 55 micron, the 10 micron aperture will read the density properly.
    I disagree, but that's an argument for a different time and place. This dive into the rat hole is probably driving the OP crazy and isn't helping him much if any.

    Quote Originally Posted by Tim Povlick View Post
    Attached is an intentionally "bad" scan I did with the Premier on Ecke 25, 4K dpi and 8 micron ap. The light and dark areas show the same general "grain size", at least to my eyes.
    What can one say to that? The laws of physics dictate that the grain clump sizes and distances must be different. Just because you can't see it doesn't mean that the scanner can't see it.

    Quote Originally Posted by Tim Povlick View Post
    FWIW I have worked with the folks at Aztek and found them to be straight talkers. This from an EE of 30 years experience that includes designs using PMT's.
    I'm not saying they are bad people. I'm just saying that some of their marketing hype is bad physics. I'm also just pointing out that people shouldn't believe all the marking hype they see -- just because somebody printed it doesn't make it fact. All marketing hype regardless of source should be taken for what it is -- an attempt to persuade you to buy something. Marketing hype is seldom an appeal to logic. It most often is an appeal to your emotions.

    So don't believe everything you read or hear. Including what you read and hear from me. Trust, but verify.

    Bruce Watson

  3. #43

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    Re: side by side comparison... large print digital back VS 4x5 color film

    It sounds like there are many parallels to digital audio at work here, but with some optical twists. In essence we are talking about the fundamentals of digital sampling. In the digital audio world an anti aliasing-filter cuts out the garbage that is out of the range of the sampling frequency. This reduces noise that would be introduced by audio signals above the sampling frequency. It seems the aperture of a scanner serves the same purpose, at least it appears so from the scans Lenny posted a link to.

    Quote Originally Posted by Lenny Eiger View Post
    It moves over a specific distance based on the ppi/aperture settings. It occurs to me I don't exactly know which one of these it chooses, but it must be the ppi if the aperture can oversample... If the slit that it looks thru is smaller than the RMS Granularity, the scanner will sample the same grain cloud more than once, creating an effect called grain anti-aliasing. If the slit matches the granularity, then you have a sharp scan with no ill effects. If the slit is larger then the image will be blurred.
    Are drum scanners oversampling up to the mechanical limits of the machine (38,000 samples for every 1/16,000th of an inch)? Where the aperture is determining the frequency of oversampling by controlling the physical size of the sample area?

    Doesn't the scanner have to be scanning every grain more than once or it would not be able to decipher individual grains. At a minimum, you must be taking samples at twice the frequency/size of the smallest grain you would like to render accurately, correct?

    Wouldn't a higher bit depth substantial increase the accuracy of scanning? 8-bits is only 256 possibilities. That is not very many in the digital sampling world. 16-bit is 65,536 possibilities. Shouldn't scanning at a higher bit depth lower the noise within the sample, as well?
    Will Wilson
    www.willwilson.com

  4. #44

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    Re: side by side comparison... large print digital back VS 4x5 color film

    Quote Originally Posted by willwilson View Post
    Are drum scanners oversampling up to the mechanical limits of the machine (38,000 samples for every 1/16,000th of an inch)? Where the aperture is determining the frequency of oversampling by controlling the physical size of the sample area?
    You're getting a little over my pay grade here, but in recent conversations about this, I believe the words "choose 8000 of these" were used. In the past, Phil mentioned that he could have supplied a 38,000 ppi scanner, but chose to do 8,000 as that was the limit of his optical resolution and he wanted to deliver "real" data.

    Quote Originally Posted by willwilson View Post
    Doesn't the scanner have to be scanning every grain more than once or it would not be able to decipher individual grains. At a minimum, you must be taking samples at twice the frequency/size of the smallest grain you would like to render accurately, correct?
    I believe this refers to something different - frequency, size, vs number of samples of the same area. Further, I asked Phil what he thought of the Nyquist theorem and whether it applied to his scanning engineering. To paraphrase it politely, he said, "No."

    Quote Originally Posted by willwilson View Post
    Wouldn't a higher bit depth substantial increase the accuracy of scanning? 8-bits is only 256 possibilities. That is not very many in the digital sampling world. 16-bit is 65,536 possibilities. Shouldn't scanning at a higher bit depth lower the noise within the sample, as well?
    I don't know the numbers for every scanner but they do vary. The older ICG's, I believe - don't quote me, did only 12 or 15 bits, etc. The Premier does full 16 bits. Since I am not clear on what causes noise in a drum scanner, I can't really answer your question. I might check in with Aztek on this.

    Lenny
    EigerStudios
    Museum Quality Drum Scanning and Printing

  5. #45

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    Re: side by side comparison... large print digital back VS 4x5 color film

    Quote Originally Posted by Lenny Eiger View Post
    I believe this refers to something different - frequency, size, vs number of samples of the same area. Further, I asked Phil what he thought of the Nyquist theorem and whether it applied to his scanning engineering. To paraphrase it politely, he said, "No."
    First of all, it's not a theorem, it's a mathematical property associated with analog to digital and digital to analog conversion.

    If it doesn't apply, then there are a few possibilities:
    His equipment is sensitive enough that it's capable of resolving smaller entities than the grains of film, and therefore is already beyond the Nyquist limit.
    It's nowhere near the Nyquist limit.
    It's magic.
    You misunderstood his response.

    I don't know the numbers for every scanner but they do vary. The older ICG's, I believe - don't quote me, did only 12 or 15 bits, etc. The Premier does full 16 bits. Since I am not clear on what causes noise in a drum scanner, I can't really answer your question. I might check in with Aztek on this.
    More bits will improve the precision of the analog to digital conversion, and make quantization errors smaller. So it should definitely lower the noise introduced through quantization error, if nothing else.

  6. #46

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    Re: side by side comparison... large print digital back VS 4x5 color film

    Quote Originally Posted by willwilson View Post
    Doesn't the scanner have to be scanning every grain more than once or it would not be able to decipher individual grains. At a minimum, you must be taking samples at twice the frequency/size of the smallest grain you would like to render accurately, correct?
    I think what you're looking for is that the smallest grains the laser can reliably resolve are 2x its wavelength.

  7. #47
    Resident Heretic Bruce Watson's Avatar
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    Re: side by side comparison... large print digital back VS 4x5 color film

    Quote Originally Posted by willwilson View Post
    Doesn't the scanner have to be scanning every grain more than once or it would not be able to decipher individual grains. At a minimum, you must be taking samples at twice the frequency/size of the smallest grain you would like to render accurately, correct?
    A drum scanner makes perfectly square pixels that are a single uniform color. A drum scanner can not decipher individual grains. They are far too small, and almost fractal in nature. Once again, see Tim Vitale's excellent paper on the topic.

    Scanners can not render grain accurately. But they do not need to.

    Bruce Watson

  8. #48

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    Re: side by side comparison... large print digital back VS 4x5 color film

    Quote Originally Posted by Rakesh Malik View Post
    First of all, it's not a theorem, it's a mathematical property associated with analog to digital and digital to analog conversion.
    Actually, it is a theorem. See "Discrete Signal Processing," Oppenheim & Schafer.
    Laurent

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    Re: side by side comparison... large print digital back VS 4x5 color film

    Oops, I thought it was a property... thanks for the correction!

  10. #50

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    Re: side by side comparison... large print digital back VS 4x5 color film

    Quote Originally Posted by Bruce Watson View Post
    You're not likely to find it in a statistics text. This is about how film works. A stochastic process implies a level of randomness with an overlay of a guiding element. In this case the randomness is the distribution of silver halides in the emulsion and the guiding element is the exposure of image itself.
    Actually a Stochastic process can be completely deterministic.

    Quote Originally Posted by Bruce Watson View Post
    To say that you can make any kind of a match between a single aperture and this range of grain clump sizes is, to me at least, quite a stretch.

    I don't follow you. You could use a statistical function to get an average or the mean maybe. Are you implying the RMS average grain size?

    I would at least not argue so much if that's what they said. But as I recall they leave out that pesky word "average."

    Should not? How about can not? I've never seen a scanner that would vary aperture during a scan. I wonder what kind of results you'd get if it did. Hmmm...
    I do not see an advantage in varying the aperture during scanning. The down side is now the sampled values represent different sized samples from the film, placing this data into a fixed 2D array of pixel elements doesn't seem correct. In thinking about this I am of the opinion fixed aperture is the way to go. Thanks for your explanation of grain and grain clumping, this is very interesting and film is certainly different from fixed array CMOS / CCD arrays in this regard.


    Quote Originally Posted by Bruce Watson View Post
    So don't believe everything you read or hear. Including what you read and hear from me. Trust, but verify.
    Agreed and thanks for the interesting discussion. I think our biggest challenge is getting the silver exposed to interesting subjects and developed correctly (at least for me).

    Best Regards,

    Tim

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