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Thread: Scanner Optical Density vs X-Ray Film Optical Density

  1. #1

    Talking Scanner Optical Density vs X-Ray Film Optical Density

    So looking for a scanner for x-ray films. Found a candidate that advertises a Dmax of 3.8

    Can anyone tell me if this Dmax optical density is the same Dmax optical density that is used to describe x-ray films?

    Understood in scanner language, Dmax is an indication of the fidelility with which a scanner will "see" all subtle tone differences, shadows, highlights, etc. The larger the Dmax, the better the fidelity of the final image.

    But for x-ray films, I think it's almost the same, but not sure. So for a developed film, the optical density range is Dmax - Dmin where an optical density of 0 is 100% transmittance (Dmin) and an optical denisty of approx 3 is 100% opaque (Dmax). So Dmax appears to not be an indication of fidelity (or sensitivity), but more an absolute measurement of opaqueness of the film?

    So if I have a scanner with a Dmax of 3.8, and I have an x-ray film with a Dmin of 0 and a Dmax of 3, my scanner should pick up all details, subtelties, tonal differences, etc from my film?

    What if my scanner Dmax was just 2? I think I would then be loosing a significant amount of information from the film...right?

    Any comments appreciated!


  2. #2
    Resident Heretic Bruce Watson's Avatar
    Join Date
    Sep 2003
    USA, North Carolina

    Re: Scanner Optical Density vs X-Ray Film Optical Density

    You know to take scanner Dmax claims with a bucket of salt, yes? Many of the manufacturers are outright lying about the Dmax their scanner can see through.

    The only way to know for sure is to test. Kodak used to make a density test target that went up to 3.6 IIRC. There are probably some of them floating around out there somewhere.

    Beyond that, CCDs typically don't have the dynamic range of PMTs. PMTs can pull the shadow detail out of trannies without much struggle, while some CCDs return just black.

    All that said, generating large Dmax with B&W film (like Xray film) is a path to unpleasant artifacts in your image. It will certainly result in bigger and courser film grain (which is how the film generates the density in the first place). It often results in an overall "cooked" look, and sometimes "density reflections" (think the ripples in a pond) in less dense regions that are next to very dense regions. Happens even with continuous agitation.

    I've done a fair amount of research into film, development, and scanning in an effort to optimize a B&W scanning workflow. All my research points me to less density instead of more. With my equipment I try for a Zone VIII density of around 1.0, which is thin enough to be difficult to print in the wet darkroom, but which scans like a dream.

    Bruce Watson

  3. #3
    Peter De Smidt's Avatar
    Join Date
    Jan 2001
    Fond du Lac, WI, USA

    Re: Scanner Optical Density vs X-Ray Film Optical Density

    Bruce is right on, in my experience.
    "Why can't we all just get along?" President Dale, Mars Attacks

  4. #4

    Re: Scanner Optical Density vs X-Ray Film Optical Density

    Thanks all for the comments. First thing I need to do is reference some of these terms Afraid I'm totally green here. Hope you don't mind if I come back with additional questions after I've educated myself a bit more on the subject.

    Thanks again,

  5. #5

    Join Date
    Jul 2007
    Austin TX

    Re: Scanner Optical Density vs X-Ray Film Optical Density

    Nancy, if you intend to scan your xray films you certainly don't want to be looking for detail in the Dmax end - it will just be buried in density and the scanner would be trying to see through it and fail - even at maximum gain setting.

    The first step would be to limit the maximum density of your film to well less than 2.0. You need to do this by controlling the exposure and development time through some initial experimentation. In fact you could consider divided development using a home brew or say commercial Diafine.

    Any scanner that is capable of a 3.0 Dmax resolution would be fine. An Epson V750 which many here use will give you a quite linear curve of Density vs 0 to 255 over the 0 to 3.0 density range.

    The B & W images I scan are developed to a density range of .05 to about 1.4, perhaps a bit denser than what Bruce mentioned above.

    So the best advice would be to focus on limiting the maximum density of your negatives if you have that ability.

    Nate Potter, Austin TX.

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