Interesting: Scanner sees densities differently

Discussion in 'B&W: Film, Paper, Chemistry' started by Usagi, Jul 10, 2008.

  1. Usagi

    Usagi Member

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    I have been using scanner as an densitometer. Before that, I used spotmeter which worked fine, but scanner is easier.

    Scanner is not linear and you have no clue what densities it gives as an output. So I checked densities that scanner gives from stouffer step tablet and created correction function that gives proper density from scanners output.

    So far, so fine...

    I used that method successfully couple of years, then I started experiments with tanning and staining developers. During those experiments and zone system calibrations for pyrocat-HD I had access to real densitometer.

    Guess what?

    It showed that all my readings were of, the more density the more error I had.
    Up to about zone V, my scanner method had given enough accurate densities, but after that - error begun to grow.

    The first I found this with tanned and stained negatives, but soon I realized that error was similar with all negatives, regardless of developers.

    So what could be reason?
    I tested stouffer step tabled with densitometer - all readings were exact.
    I have correction function for scanner which gives exactly right density readings from stouffer tablet.

    But from negative, it is different. Why? Does scanner see the density of silver film somehow differently than test tablet?
     
  2. gainer

    gainer Subscriber

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    Photographic density is the logarithm of the ratio of input illumination to output illumination. If you plot your results on semilog graph paper or the log of your results on ordinary graph paper, you may see the light. Your spot meter shows a number proportional to the logarithm of illumination of its sensor.
     
  3. Usagi

    Usagi Member

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    Yes, I know that density is logarithm.
    But that does not explain why I get different density reading from b/w film than calibration step tablet.

    In my best knowledge, both should pass equally light at when both have equal density.

    For example, when transmission step wedge 'step' have defined density 1.25, it gives real density 1.26 with densitometer and 1.44 with my scanner.
    With that information I know that when scanner's densitometer gives reading 1.44, it is actually 1.25.

    I have even created correction table. The Y axis is density given by scanner and the X axis is real density (measured from stouffer transmission step wedge).

    This is really accurate with transmission wedge. Thus, it should also be accurate with any kind of transmissable material.
    But for B/W film it seems not to be. For example, Neopan 400 film, it gives density 1.25 + FBF for zone VIII, which sounds really good. But real densitometer gives 1.17 + FBF.

    Why such difference? Both equipments, the scanner and the densitometer reads transmission wedge accurately (ofcourse, scanner after applying correction). So why not negative?
     
  4. David A. Goldfarb

    David A. Goldfarb Moderator Staff Member Moderator

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    What developer are you using for your film? If you are using a staining developer, you may get different densities, depending on how you measure. If the densitometer has a UV channel, for instance, you might get a higher UV density than a white light density from a scanner. Or if you scan in RGB, the green channel might have a higher density than the other channels. And then the fluorescent light source on the scanner has a different spectral output than the light source in the densitometer, so that could also cause differences in readings with a staining developer.
     
  5. RobC

    RobC Member

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  6. Photo Engineer

    Photo Engineer Subscriber

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    No silver image from a standard developer is precisely neutral, so unless the densitometer and the scanner match in spectral response when reading, there are apt to be differences.

    We all know that you can get warm or cold tones from different developer/emulsion combinations and the two instruments can read densities on these quite differently.

    PE
     
  7. Usagi

    Usagi Member

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    Thanks PE,
    that's probably the reason.

    I have tested my "scanner densitometer" with various developers, Pyrocat-HD, DiXactol, Rodinal, Xtol.
    Even tested with cokin ND-filter - which I found not to be very neutral...

    So my final thought is that transmission wedge can be used to create calibration function for scanner, and then scanner can be used as densitometer. At least it shows about where the densities are.

    But how are the real densitometers? There are lot of folks that uses densitometers for creating characterist curves, calibrating zone systems, .... Are they aware that densitometer may not be really accurate with used material (film + developer combination)?

    Mostly people are aware that staining developers need to be measured with blue light or better, with UV. But for other materials - I really don't know.

    Sensitometry seems to be can of worms... That I just opened and lost the lid.
     
  8. Usagi

    Usagi Member

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  9. Photo Engineer

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    Jukka;

    It is more like opening a can of live grashoppers. :D

    PE
     
  10. Saganich

    Saganich Subscriber

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    The list of variables which could cause such a difference is quite long but it is likely that the point light source of the densitometer scatters within dense negs differently then a diffused light source of a scanner. This could be exacerbated by different film types, grain patterns, stain, etc. One way to check this is to make the densitometer see like a scanner. Some densitometers (older macbeth's) have mesh filters under the plastic base which can be adjusted between the point source and aperture which will diffuse and flatten out the spectrum.
     
  11. Photo Engineer

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    Yes, a densitometer uses a different light source and collector than a scanner does, and this changes the way it sees light. This is especially true of reflection density.

    PE
     
  12. gainer

    gainer Subscriber

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    You must face the fact that a densitometer compares two light intensities, the incident and the transmitted. All it knows about the colors of the two intensities is..? Let us suppose that the sensor of the densitometer responds equally to each wave length of the visible spectrum. Unless your printing material has the same response spectrum, you cannot learn from the densitometer what will be the effect of a certain measured light value on your printing material. You could measure a very bright red light that would have no effect on the paper. Density measured with a very narrow light spectrum may tell you little about the printing density of a negative that appears to the eye to be neutral gray.

    I have a home designed and built easel exposure meter that I can calibrate to read projection density as long as I am measuring a negative developed in a certain developer. If I change to a different developer, especially if I change to a staining developer, I must recalibrate both amplitude and gradient if I expect to be able to predict the density that will be printed at a measurement point. It is especially difficult if I am using VC paper whose contrast varies with color of light. There are many times when a simple test strip is more valuable than any meter.

    You may not get the expected result from anything less than a spectrum analyzer.
     
  13. Sparky

    Sparky Member

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    I suspect you are making far too many assumptions in your 'experiments'. i.e. are you always using the same wavelength of light for your tests? Or are you using multiples? What control have you designed in for light scattering due to reading a particulate/composite image like film vs. the step tablet? What control have you made to compensate for the difference in light source/transmission between the densitometer and scanner. It seems these are just a few factors that can affect your outcome significantly!

    I'd probably start over making your own step tablet with the same film - that way you can control for any differences in the materials.
     
  14. Photo Engineer

    Photo Engineer Subscriber

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    Well, here is an interesting experiment.

    Read a print with a step scale with a good densitometer.

    Read it again under viewing conditions with a spot photometer calibrated to read the densities the same as if it were the densitometer. In other words, use it as a calibrated eyeball.

    The results are striking and very surprising. Even with the best matched units under identical illuminants, the results are different.

    PE
     
  15. RobC

    RobC Member

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    Only surprising if you have made bad assumptions without understanding how a densitometer and light meter work.
     
  16. Photo Engineer

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    A spot photometer isn't a light meter. They make units that supposedly read like either reflection or transmission densitometers. MacBeth makes a "viewing chamber" to simulate the standard viewing condition or the standard integrating sphere of a densitometer.

    Thus you can effectively duplicate a densitometer in the open air and then measure things visually and with an instrument.

    PE
     
  17. RobC

    RobC Member

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    So how does it know what the contrast ratio is and how does that compare to the contrast ratio of the densitometer? That is, a densitomter can be zeroed on its light output so readings are relative to the light output. What do you zero the hand held densitometer on so that you know what the relative brightness is?
     
  18. Photo Engineer

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    The Macbeth unit could be coupled to the Macbeth densitometer for calibration. I have to admit that I never calibrated one. I was just given the calibrated unit and taught how to use it in conjunction with the densitometers. We then could correlate what the eye "thought" it saw, vs what the instruments read.

    The calibrated viewing cabinet was about the size of a modern home buffet or about 7 ft high and about 6 feet ling and about 3-4 feet deep. An adjustable overhang allowed for the different lighting conditions to match the densitometer integrating sphere.

    The photometer was mounted at standard viewing distance from a print at an angle of 45 degrees. The densitometer collected light at about 45 degrees as well.

    This data was all suplemented by use of a goniophotometric reading to explain what was going on.

    Basically, a densitometer integrates all reflected or transmitted light, but the eye only sees from one perspective over about 3 degrees to 5 degrees or so. This difference relates to the surface characteristics of the print and causes variations in density as a function of viewing angle that straight densitometry does not pick up even though the instrumentation is identical.

    The same is true of a scanner BTW.

    PE
     
  19. Photo Engineer

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    I've thought of a better way to describe this!

    On a sheet of graph paper, imagine a teardrop. The X axis is angle and the Y axis is density. This is a representation of a plot using a goniophotometer. It measures density vs angle of view. Now, imagine a set of teardrops drawn inside of each other and getting smaller. This is the characteristic curve read by a goniophotometer.

    Now, imagine that a scanner looks from the point of the teardrop down. One point source down through the material (and back up if it is a print).

    The eye and the photometer look at a 3 - 5 degree segment of the side of the teardrop at the viewing angle.

    The densitometer is the value obtained by integrating the entire teardrop into a value read through an aperature usually placed at a fixed angle such as at 45 degrees.

    Even if all is calibrated to read all densities as being equal they still get different answers.

    Hope this helps with the above.

    PE
     
  20. Usagi

    Usagi Member

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    I will continue this old thread :smile:

    rob champagne posted that interesting link http://www.unblinkingeye.com/Articles/Densi/densi.html

    I guessed that it can be used as correction table between visual and uv densitometers.
    But will it really work that way?

    For example, if I get visual densities for film X developed in Pyrocat-HD and got reading 1.10, can I convert it to UV reading with using that table (for this example, result would be 1.59).

    As the sensitometry has turned out to be not so accurate, is the error caused by using such table for correction tolerable. I don't need really exact readings, just reading for calibrating my N, N- and N+ developments.
     
  21. Nicholas Lindan

    Nicholas Lindan Advertiser Advertiser

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    > I have been using [a calibrated] scanner as a densitometer ... then I started experiments
    > with tanning and staining developers ... all my readings were off ...

    Sensitometery/densitometry for printing negatives developed with staining developers is, as you have stated, a can of worms: extra large, extra wiggly and extra slimy worms, all in an extra large can.

    B&W densitometery makes the assumption that the negative attenuates all wavelengths of light equally. For most black and white films this is a valid assumption. The reading offsets caused by the spectral variations of the densitometer light source, the densitometer sensor response, the enlarger light source and the paper spectral response can all be corrected for with a linear calibration factor - and/or all come out in the calibration process.

    Color densitometry - for color printing - works by matching the color filters in the densitometer to the spectral sensitivity of the dyes in the color emulsion. Color densitometers are made for a specific mini-lab or process. They come with filters that are specific for the materials used by the laboratory and with a light source that mimics the light source used in the printing machine. They are designed to read quality control strips where there is a nice big patch of even density to read.

    Color densitometers for graphic arts use are matched to RGB separation filters and CMYK printing inks. Again, they read QC patches or the patch left behind by imaging a step tablet.

    UV densitometers are designed for graphic arts work with mercury/metal halide UV light sources and polymer/gum emulsions.

    Densitometers use a diffuse light source for transmission and an off-axis light source for reflection: both are specified in ANSI/ISO standards. These standards are designed for use in graphic arts/printing but are often used in color densitometers for mini-lab use simply because they provide some interoperability.

    Densitometers are _not_ designed for use in conventional darkrooms. However, for black and white work with regular silver processes they work well enough to be useful. For alt-process work densitometers must be used as they are the only game in town. UV graphic arts densitometers work well with many UV sensitive alternative materials.

    Scanners are even further removed from darkroom applications, though if used with the most conventional of black & white materials and processes it is possible to calibrate them so useful results can be obtained.

    The proper instruments to use in a traditional enlarger-based darkroom are color analyzers and enlarging meters.

    By working under the enlarger they automatically take into account the effect of Callier effects from condenser and diffusion light sources, flare and stray light of the enlarger. Many of the spectral effects of the light source are accounted for in the calibration.

    The measuring spot of an analyzer or meter is small enough to read millimeter sized highlights. The same highlights on the negative are only a few hundreds of microns across and can't be read with a bench densitometer.

    A color analyzer will compensate for the spectrum of the enlarger light source because it has filters that match the sensitivity of the color emulsions in the paper. An enlarging meter will compensate for the enlarger's lamp spectrum with a constant calibration factor: it either comes out in the individual calibration of a meter and enlarger or can be added to a supplied calibration developed on a different enlarger.


    The premise of all this goes straight out the window when staining developers are used.

    Conversely to black and white work, the color of the negative now comes into play - and it interacts with the spectra of the light source, the color response of the measuring instrument, the spectral sensitivity of the paper, any contrast filtration, and the effect of spectrum on multi-grade papers.

    Conversely to color work, the color response of the densitometer is not matched to the color response of the paper. Some users of graded paper have had limited success using the blue channel of a color densitometer (yellow channel on a color analyzer) to read pyro negatives.

    An enlarging meter can be used with staining developers, but the methods needed are different from those used with conventional negatives. The stain produced is proportional to silver density and so a multiplicative correction factor is needed: the meter reads a density of 2 stops (0.6 OD) but the paper may see this as a density 1.5 times higher at 3 stops (0.9 OD). Changes to the lens aperture or the enlarging height, however, are seen the same by the paper and the meter.

    For this reason, a meter needs to be used first in absolute mode to read the illumination of the negative's B+F (no stain) on the easel to take into account light source intensity, lens aperture, enlarging ratio and negative base density. The meter is then used in density mode to determine paper grade, exposure needed to bring the low values (shadows) to the desired density and the exposures needed for burning and dodging.

    The Darkroom Automation meter (disclaimer: I own the company that makes Darkroom Automation products) will work in absolute/density mode. However, in my experience, the calibration procedures for using the meter with pyro in combination with MG paper may not be worth it for most casual pyro printers - it is often easier to make the usual split grade printing double test print.
     
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  22. Nicholas Lindan

    Nicholas Lindan Advertiser Advertiser

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    > the more density the more error I had.

    The more density, the more stain, the more error.

    Even with unstained negatives the discrepancy between density measurements will increase with density.

    Black and white negatives have a color component that is driven by the emulsion particle size. Very fine grained emulsions absorb more in the blue than their coarser grained relatives. Many developers, though not staining, do still produce a small amount of stain and a good densitometer will pick this up.

    Additionally, the differences introduced by Callier effects, densitometer optics and stray light will increase with density.

    There is an old saying "A man with more than one watch never knows what time it is"; the same holds true of densitometers. Pick one, ignore the rest. For enlarging, you are best off with an enlarging meter or analyzer.

    > Does scanner see the density of silver
    > film somehow differently than test tablet?

    Common Stoufer-style 'step tablets' are made from conventional silver film. In the old days step tablets/wedges were stepped wedges of colored glass or gelatine.
     
  23. Usagi

    Usagi Member

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    Thank you :smile:

    This thread has been very informative to me. I have learned that there are way too much wrong information about densitometers laying in the books, internet talks etc. Even on workshops. The common mistake seems to be that densitometer is absolytely right - at all situations and with all materials.

    That made me thinking whole thing again. Do I really even need sensitometric approach?
    The answer is probably 'no'.
    Yes, the characteristic curves are nice and sometimes useful for estimating developing times for different contrasts.
    But for accurate enough calibration (zone system in this case) the plain old calibration by printing is good.

    So I will continue my experiments with staining and tanning developers by finding speed point and developing times for different contrasts simply by doing prints from test strips.

    By the way, that was my approach for years during early nineties before I shifted to sensitometry.