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  1. #11

    Join Date
    May 2008
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    Sunnyvale, CA
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    35mm
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    My humble Epson 4490 works quite well as a densitometer over the density range I need, 0.15 to about 2.2. I did an initial calibration by scanning a pre-calibrated step wedge and creating a Photoshop curve to convert scanned values to densities. Scanned densities agree with a dedicated film densitometer to within +/- 0.02 which is plenty for me. Density readings from the scanner are repeatable to within the same tolerance (+/- 0.02) as long as I let the scanner warm up for about 20 minutes first.

    As others have said, it is critical to turn off all auto functions. I use VueScan's raw mode.

    I scan at 16 bits per channel to capture enough bits in the densest areas (lowest scanned values). Eight bits per channel won't cut it. Applying a gamma=3 in Photoshop before applying the calibration curve expands the range of the lowest scan values (greatest density) and compresses the highest values (least density). Otherwise the low values are too close together for a good calibration curve. I set up my curve so that pixel values of 0-255 represent densities of 0.00-2.55 (although the scanner doesn't actually reach the extremes). I have a Photoshop file with the layers ready to go so I just copy in the scanned image and read out densities.

    I also have Photoshop layers that convert negative density to print values for various contrast filters based on a standard printing exposure. I simulate different printing exposures by adding or subtracting a constant to density values. For example, adding +30 to all pixels is equivalent to a density increase of 0.30 which is one stop less printing exposure. This method helps me visually come up with a good starting point (contrast filter and printing exposure) to make better use of precious darkroom time. It was a bear to set up but comes in handy now.

  2. #12
    RalphLambrecht's Avatar
    Join Date
    Sep 2003
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    the villages .centralflorida,USA and Germany
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    Quote Originally Posted by glarsson View Post
    My humble Epson 4490 works quite well as a densitometer over the density range I need, 0.15 to about 2.2. I did an initial calibration by scanning a pre-calibrated step wedge and creating a Photoshop curve to convert scanned values to densities. Scanned densities agree with a dedicated film densitometer to within +/- 0.02 which is plenty for me. Density readings from the scanner are repeatable to within the same tolerance (+/- 0.02) as long as I let the scanner warm up for about 20 minutes first.

    As others have said, it is critical to turn off all auto functions. I use VueScan's raw mode.

    I scan at 16 bits per channel to capture enough bits in the densest areas (lowest scanned values). Eight bits per channel won't cut it. Applying a gamma=3 in Photoshop before applying the calibration curve expands the range of the lowest scan values (greatest density) and compresses the highest values (least density). Otherwise the low values are too close together for a good calibration curve. I set up my curve so that pixel values of 0-255 represent densities of 0.00-2.55 (although the scanner doesn't actually reach the extremes). I have a Photoshop file with the layers ready to go so I just copy in the scanned image and read out densities.

    I also have Photoshop layers that convert negative density to print values for various contrast filters based on a standard printing exposure. I simulate different printing exposures by adding or subtracting a constant to density values. For example, adding +30 to all pixels is equivalent to a density increase of 0.30 which is one stop less printing exposure. This method helps me visually come up with a good starting point (contrast filter and printing exposure) to make better use of precious darkroom time. It was a bear to set up but comes in handy now.
    Sounds like a good approach.
    Regards

    Ralph W. Lambrecht
    www.darkroomagic.comrorrlambrec@ymail.com[/URL]
    www.waybeyondmonochrome.com

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