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  1. #1

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    Contrast Curves using VC Paper

    I recently purchased a densitometer (Xrite-810) to get a better handle on the relationship between my exposure, negatives and prints. I also purchased a Stouffer step wedge that I used to make prints on Ilford MGIV Deluxe RC Pearl paper using Ilford VC filters. Development was done in Dektol 1+2 20C and 1m30sec development. When I measured the results I was a little surprised in that the contrast in the hi-light region appeared to be the same in grades 1,2 and 3 with the only separation being in the shadows. Attached is the data for both a LED Diffusion head and Tungsten Condenser (75W bulb). Both are quite similar. The densitometer checks out vs it's calibration material (reflected metal piece and the transmitted step wedge). I also measured the Stouffer step wedge using the densitometer. Do these results seem typical? They do not seem to match the graphs that Phil Davis has in his BTZS book. Then again, I think he was using single graded paper. My concern is that in practice the shadows will print at a higher relative contrast vs the highlights.
    Attached Thumbnails Attached Thumbnails Diffusion.jpg   Condenser.jpg  
    Regards,
    Jim

  2. #2

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    You have your Density data plotted correctly, but you have the Density vertical axis labeled as 'Log' - which it isn't (and shouldn't be).


    Such plots are referred to as D - log E plots. ( Density-Versus-the-Log-of-Exposure )

    BTW, the condensor plots look OK to me. Plot the three different grades on separate sheets of paper so you can slide them back and forth to more easily see the differences.
    Last edited by Prof_Pixel; 03-04-2013 at 06:45 PM. Click to view previous post history.

  3. #3

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    Thanks Prof_Pixel. Both axis would be a labeling issue, the numbers are the densitometer readings. Horizontal in transmitted mode, vertical in the reflected mode.

    I see what you mean, if I align by a mid tone, then the curves would indeed look better. At least for the condenser / tungsten bulb. When I look at the LED source across grades 00 thru 5 and align by the mid-tone, I see the curves again make more sense. There seems to be a "kink" in the LED curve (around a print density of 0.75) that I would like to understand better. Could this be an effect of absorption between the blue and green LED's in regards to the paper? Could an additional filter under the light source help smooth this out?

    I suppose I should repeat the experiment to see if I get the same result.
    Last edited by jlpape; 03-04-2013 at 07:27 PM. Click to view previous post history.
    Regards,
    Jim

  4. #4
    Stephen Benskin's Avatar
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    Did you mask the step tablet?

  5. #5

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    No, I did not need to. I inserted the 4x5 step tablet in the enlarger and filled the field of an 8x10 sheet of paper.
    Regards,
    Jim

  6. #6

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    Interesting measurements! But not so surprising finding, when you remember that variable contrast is achived by superposing (adding) the densities of 2 (3 for some papers) layers: one soft, the other hard contrast; at the highest print densities, the hard layer dominates the response. I had assumed that paper manufacturers had somehow addressed that issue; it does not seem to be the case, unfortunately.

    This said:
    (1) presumably you placed the step wedge in the negative carrier, not lying on the paper; since you see different responses for different heads, I assume that is what you did;
    (2) if you want to address the response curve of the paper, and isolate it from the response of the system (projection system + paper) you should plot the paper density versus the (log)illumination measured on the enlarger board, on the projected image of the step wedge. Otherwise, you are measuring a combination of the paper response and of the effective density of the step wedge, for which the Calllier effect depends on the illuminant. Alternatively, you might place the step wedge in contact with the paper; that would eliminate the role of the head (condenser versus diffusion) but would still not guarantee that the effective density (seen by the paper) is the same as measured by the densitometer.

  7. #7

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    Thanks bernard_L, as Prof_Pixel points out, the condenser / tungsten curves look reasonable when you align the curves by the mid tone. I downloaded the tech sheet from the Ilford web site for MGIV RC and it contains a similar plot, showing that my results are quite similar to the tests done by Ilford. So I am satisfied that my measurements and values I obtain are reasonable.

    The Diffusion / LED source has me interested. There is a more pronounced "kink" in these curves that leads me to believe that I am getting a substantially different contrast for the shadows than I am for the highlights. I need to absorb your comments a bit more, but I was starting to look at the spectrum of the LED vs a tungsten source to see if this could explain the difference. A tungsten curve is fairly constant in the 400-600nm wavelength as opposed to the LED that has some sharp peak's and valley's. Could some of the "missing" wavelengths cause this? Maybe I guess I would need to compare the spectrum to the pass wavelengths of the filters. I am also going to generate curves for non-VC grade 2 & 3 papers using the diffusion/LED head and condenser / tungsten head. My guess is that they will look a lot more similar to one another.
    Regards,
    Jim

  8. #8

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    I was starting to look at the spectrum of the LED
    Good point! MGIV has three emulsions since 1984; see:
    http://www.photomemorabilia.co.uk/Il...ultigrade.html
    and possibly the response of the "medium-hard" emulsion falls in a hole of the LED spectrum. See:, e.g.
    http://www.ecse.rpi.edu/~schubert/Li...%20spectra.jpg
    http://www.ecse.rpi.edu/~schubert/Li...%20fitting.jpg
    Then you are left with only the two extreme emulsions, which makes more evident the inherent problem of variable-contrast paper, when the curves of individual emulsions don't merge well into the equivalent of a single-grade paper.
    So much for my (vague) plans to try an LED lamp in my enlarger.

  9. #9
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    See:

    http://www.darkroomautomation.com/su...vcworkings.pdf

    In general, changes in contrast in the lower grades are in the shadows while the contrast changes in the higher grades are in the highlights.

    The 'bump' in the middle of the lower VC grades is an intrinsic property of VC paper. The difference you are seeing is because the two heads are printing at slightly different contrasts because they have different spectral curves.
    DARKROOM AUTOMATION
    f-Stop Timers - Enlarging Meters
    http://www.darkroomautomation.com/da-main.htm

  10. #10

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    I got hold of some graded paper (Ilford Ilfospeed Grade 2) and ran the same test. Sorry for not labeling the axis horizontal = step tablet density, vertical = paper density. The red curve is the graded paper, blue the VC paper printed with a grade 2 filter. Seems to confirm that the spectral distribution is playing a role.

    I am going to start using graded paper and will optimize my exposure and development around this.
    Attached Thumbnails Attached Thumbnails Graded vs VC.jpg  
    Regards,
    Jim

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