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Thread: I'm affraid it won't be long

  1. #341

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    Re: I'm affraid it won't be long

    [QUOTE=timparkin;1020120]Yeah that's fair enough Working with a fixed MTF50 does give a universal reference point but I do honestly think that MTF10 or even MTF5 would be more realistic when comparing the visual resolution of 'system'. However in reality it's probably productive to show both.

    You have an interesting point here that I've not fully contemplated. As the spacial frequency of an array of lines and spaces increases The MTF drops (I think more or less linearly). At some point it approaches near zero at some high frequency which means the utility of dynamic range is near zero for that high frequency end point and one can barely tell a space from a line. Full DR is non recoverable from a piece of film with finest lines and spaces. The same holds for actual images. Using MTF10 or MTF5 also will result in reduced dynamic range for fine features. In fact using MTF50 will show a slight reduction in dynamic range for that spacial frequency. I think it is true that the full dynamic range as a function of frequency can only be had when MTF is 100% for a set of line pairs.

    So we have a sliding scale of dynamic range starting at 100% at some low frequency (large linewidths) and ending at near 0% for some high frequency (small linewidths). Fine detail is washed out on critically sharp prints at low MTF.

    Nate Potter, Austin TX.

  2. #342
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    Re: I'm affraid it won't be long

    Quote Originally Posted by Arne Croell View Post
    The "molecular radius" of silver bromide does not matter at all (nor the lattice constant, which is 0.57745 nm for pure AgBr if that was meant), it is the size of the crystal grain in the emulsion. The size of the original silver halide crystal is related to the size of the "grain" that we later see, but it is not the same. If a silver halide grain gets enough exposure (usually at least 4 photons) it will be completely changed to silver in the development process. That silver is usually a bunch of silver filaments whereas the original grain was either something close to a cube shape or triangular to six-sided plate in the case of T-grains. What we see in the end as "grain" in the print, are the holes between the silver filaments in the negative.
    Silver halide grains have a pretty wide range of sizes depending on the film. It ranges from about 0.03-0.04Ám (30-40nm, http://www.ilfordphoto.com/holofx/holofx.asp) for holographic film with a claimed resolution of 7000lp/mm (so-called Lippmann or micrate emulsions) to T-MAX 3200 with a grain width and length of 5-6Ám (thickness of 0.2Ám), which is about the pixel pitch of a D800 sensor.
    If the molecular radius (I used silver chloride as it was one of the first sensitizing compounds used in photography and dates back to the 1830's) is not relevant as you say, then is the pixel radius also not relevant? If the "grain" visible in film are "holes between the silver filiments in the negative," are there corresponding "holes" in the digital equivalent? If so, how do they compare in size? Finally, how does the inter-molecular distance compare with the inter-pixel spacing?

    Thomas

  3. #343

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    Re: I'm affraid it won't be long

    Quote Originally Posted by tgtaylor View Post
    If the molecular radius (I used silver chloride as it was one of the first sensitizing compounds used in photography and dates back to the 1830's) is not relevant as you say, then is the pixel radius also not relevant? If the "grain" visible in film are "holes between the silver filiments in the negative," are there corresponding "holes" in the digital equivalent? If so, how do they compare in size? Finally, how does the inter-molecular distance compare with the inter-pixel spacing?

    Thomas
    Well I'll chime in and hope that Arne says about the same thing with a lot less words.

    A number of people have suggested (even on this forum) that it would be interesting to compare silver grains in an emulsion to pixels in a digital sensor. One could say there are some similarities. But photon capture is quite fundamentally different. As Arne already pointed out a silver halide crystal contains many silver halide molecules so it's best not to talk about molecules and their nano size but to talk about crystals which are say submicron to perhaps several microns in size depending greatly on the type of film. How much of the grain gets converted to silver during exposure depends on the exposure dose. The silver content is amplified during development sometimes thousands of times. Generally the size of the silver grain ends up much larger than the precursor crystal. For ordinary films in use here the range may be 0.1 Ám to say 20 Ám.

    Now in silver emulsions there are spaces between silver grains but in regions of high density these grains overlap each other while in low density regions there can be more space than silver. To characterize the way light moves thru a silver emulsion one can employ mathematical percolation theory but such contortions are irrelevant for a simple discussion here (and surely irrelevant for making photographs).

    In the case of a digital sensor we have a regular array of photosensitive pixels where the incoming photons generate hole/electron pairs in fact without the multiplication one gets during silver development with film. Of course the hole/electron pairs are swept under an electric field to a positive or negative terminal in the pixel where the holes are recombined and electrons are forced to flow in an external circuit. Except for resistive losses and unproductive carrier recombination, the incoming photon flux is proportional to the current that can be extracted. The size of common sensors now in use run from about 5 Ám to 10Ám. They do not overlap, are fixed in space and are separated from each other by a dead space. The space between pixels is needed for circuitry and might range from 2Ám to 5 Ám. Currently there is considerable research effort to apply non reflective coating to the space between the pixels in order to reduce spurious light reflection.

    So there is a really fundamentally different recording method between silver halide and a digital sensor. As the sensor is made smaller there is less area of each pixel in which to absorb photons so maximum signal to noise floor is progressively limited. Due to a generally huge multiplication of silver atoms during development of film the density range can be very high from film base to max density, perhaps up to 12 stops (often more than is useable) so film addicts need to watch Dmax during development.

    At this point in sensor development a pixel approaches the dimensions of a large silver halide crystal around 5 Ám. (.005 mm). But silver halide emulsions will have a range of crystal sizes down into the submicron region and special fine grained film into tens to hundreds of nm range which leads to a finer microcontrast structure.

    So silver halide emulsions are really a continuous and random array of crystal receptors of different dimensions. Sensors employ a regular array of discrete pixels with regular spacings.

    This all leads to quite different micro detail between the two but sometimes not so obvious when studying side by side prints made by each method.

    Nate Potter, Austin TX.

  4. #344
    the Docter is in Arne Croell's Avatar
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    Re: I'm affraid it won't be long

    Quote Originally Posted by tgtaylor View Post
    If the molecular radius (I used silver chloride as it was one of the first sensitizing compounds used in photography and dates back to the 1830's) is not relevant as you say, then is the pixel radius also not relevant? If the "grain" visible in film are "holes between the silver filiments in the negative," are there corresponding "holes" in the digital equivalent? If so, how do they compare in size? Finally, how does the inter-molecular distance compare with the inter-pixel spacing?

    Thomas
    What Nate said. When I said the "molecular radius" does not count, I meant that in relation to the use of the term. It implies that you are looking at atomic distances in AgBr or AgCl, sub-nanometer, and those don't count. Even the smallest silver halide crystals ("grains") of 30nm length will contain about 500000-600000 AgBr or AgCl molecules (actually ions). The size of those crystals, however, counts.

  5. #345
    Large Format Rocks ImSoNegative's Avatar
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    Re: I'm affraid it won't be long

    Wow we have come a long way with this thread. first post : "I'm affraid it won't be long before I leave my treasured 4x5 at home on my trips to Yosemite. My digital has so much more resolution and sharpness that I don't see the need in lugging around 25-30 pounds of camera gear anymore.When I make 16x20's from my digital files there every bit as good if not better than my 4x5 negs.I'm going to Yosemite in May and will bring my 4x5 but I'm only bringing aroung 6-7 holders instead of my usual 12-15 holders.It's sad but it's the new reality.I'll miss composing the image on the ground glass and setting the fstop,shutter speed and I miss pressing the cable release.About the only thing I won't miss is people asking me "what kind of camera is that".I wonder if the digital age was around in the mid fifties would Ansel Adams had left his 8x10 at home?

    Last post "What Nate said. When I said the "molecular radius" does not count, I meant that in relation to the use of the term. It implies that you are looking at atomic distances in AgBr or AgCl, sub-nanometer, and those don't count. Even the smallest silver halide crystals ("grains") of 30nm length will contain about 500000-600000 AgBr or AgCl molecules (actually ions). The size of those crystals, however, counts. "

    and what about the flux capacitor?
    "WOW! Now thats a big camera. By the way, how many megapixels is that thing?"

  6. #346
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    Re: I'm affraid it won't be long

    i don't think it really matters what you use,
    as long as you are enjoying yourself.

    sometimes i forget to load film
    and i haev just as good a time as i would have had with it ..
    its just the photos are in my head, instead of on film and paper ...

  7. #347
    Jac@stafford.net's Avatar
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    Re: I'm affraid it won't be long

    Quote Originally Posted by tgtaylor View Post
    If the molecular radius (I used silver chloride as it was one of the first sensitizing compounds used in photography and dates back to the 1830's) is not relevant as you say, then is the pixel radius also not relevant? If the "grain" visible in film are "holes between the silver filiments in the negative," are there corresponding "holes" in the digital equivalent? If so, how do they compare in size? Finally, how does the inter-molecular distance compare with the inter-pixel spacing?

    Thomas
    Just my two-bits (10) - What one sees in digital as 'grain like' is noise, and it is white. Grain is black. Grain is the very fundamental part of silver-based photography, it is what makes the picture while digital noise is is a exception to digital's goal.

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