Densitometery and Contrast Index (CI)

Discussion in 'Alternative Processes' started by photomc, Apr 1, 2005.

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  1. photomc

    photomc Member

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    As the new owner of a densitometer, attempting to get a better understanding of how it is used to determine the contrast index (CI) of a negative. Can someone point me to a resource that might help explain how the CI is determined or perhaps offer a little Densitometery for Dummies...

    Thanks,
     
  2. Jorge

    Jorge Inactive

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    Hate to sound like a broken record but the best way to figure this out is to read Phil Davis' book "BTZS".

    Calculating CI or Gamma as Kodak or Ilford does it is somewhat involved and requires strict controls. If you are still curious about how they do it you can read Todd and Zakia'a "Photographic Sensitometry." But it is really dry and overkill if all you want is to use the results to better control your process.
     
  3. photomc

    photomc Member

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    Not a broken record at all Jorge, guess just need to go ahead and order the BTZS book and read it...sometimes I think the only dense thing around the house is me :smile:

    But...even old dogs learn new tricks sometimes. Main thing is not sure I understood what part of a negative is read when I see posts here about negative of 1.6 or 1.7..
    so I would guess that reading the book should help clarify.

    Thanks....BTW, if I could would ship paper and metal to you just to see some new prints....
     
  4. Jorge

    Jorge Inactive

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    You are confusing density range (DR) with CI. Contrast Index, to explain it simply is the slope of the straight part of the H&D curve. The trick, and where manufacturers dont agree is what two points to use to determine the slope. So Kodak does it one one, Ilford does it another way, but they are both pretty close.
    Density range is the spread between the shadow with detail and the highlight with detail as read with a densitometer. For example, if your shadow with detail has a value of .45 and your highlight has a value of 1.9 then your density range is 1.45....

    Thanks for the offer Mike, I might take you up on it and have you help me reship paper, ordering straight from B&S is just too expensive and they refuse not to include an invoice which has shipping charges on it. Mexican customs bases their import tax on the price of the item plus the shipping charge.....what a scam!
     
  5. rbarker

    rbarker Member

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    Sounds like we need to get a couple of cross-border "underground railroads" going. Take Ilford film north to Canada, return with medications. Take alt-process paper south, return with the really good tequila. And here I thought NAFTA was supposed to solve all of that. (ROFLMAO)
     
  6. Jorge

    Jorge Inactive

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    Aww men, dont get me started with NAFTA.....Mexicans really got suckered with that deal.....BTW, medicines are even cheaper here than they are in Canada....
     
  7. noseoil

    noseoil Member

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    As to a trans-border underground railroad, you're just a bit too late. The customs service keeps finding tunnels which go under the border. Unfortunately, there is better money in drugs and humans than in photographic paper and supplies.

    Jorge, what kind of import duties are you paying down there? "Isn't there some way we can resolve this problem?" tim
     
  8. colrehogan

    colrehogan Member

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    Glad you asked the question, Mike. I've been having the same problems. I have read that part of the BTZS book, but don't understand all that it says yet. I'll have to keep working on it.
     
  9. ann

    ann Subscriber

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    check with Lee, i sent him some stuff that Richard Knoppow sent to me, it might help.

    You are not the only one tht stuggles with this lol
     
  10. photomc

    photomc Member

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    LOL - Not surprised Jorge, am the first to admit that I am confused. :smile:
    But then, just spent the past week in a class on object oriented analysis and design (geek speak) - three semester's in a 5 days..whew!

    If I understand what you are saying, is the DR is you take a densitometer reading of the shadows of the negative (my guess would be the one you metered when you determine the exposure), then read the highlights (same as used for the inital metering) and then do the math to find you DR. Right?
    From what I have read, this is also, how you BTZS guys read a scene to get your SBR - Ok, there is more to it than that, but this is part of it - right?

    Not a problem buddy, would be happy to help out any way I can. Buying materials should not be this much a PIA, no matter where you live.

    Thanks Diane, it really helps to know I'm not the only one that is confused.
    But with all the good folks here, I know we will get this down and get on to more serious things like shooting/printing and of course hanging these nice new prints on the wall...after all the main reason I am doing this is so that I can put something up I can really be proud of.


    Thanks Ann, will be visiting with Lee at the DR - with densitometer in hand later this morning, your timing is perfect.


    You know if I did not think this was all worth it, I really would not do it...but have seen enough difference in peoples work here before and after they tamed the process that I firmly believe it is worth the effort.

    Thanks to everyone for you input...
     
  11. smieglitz

    smieglitz Member

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    Get ye some standard graph paper and scale the vertical axis for density in 0.10 intervals. For the horizontal axis have a spacing interval of one stop (zone) equal to 0.30 density units. From the same graph paper make a ruler scaled identically as the vertical axis. Make an index mark at the edge of origin of the scale and mark it "A". Go to the 0.20 value and make another mark labelled "B" at the edge. At a value of 2.20 make a mark "C". (I'm relating this from memory but the last value is 2.20 IIRC. Check this in the Kodak or Ilford literature or perhaps someone else could confirm?)

    Once you plot your characteristic curve of film test densities vs. exposure, determine the level of fbf and draw a horizontal line through the (0.00 net density) FBF value parallel to the horizontal axis. Take the CI index ruler you've made and position point A on that horizontal line. Slide the ruler along that horizontal line pivoting the ruler until points B & C both lay on the characteristic curve you've drawn. When you have A on the horizontal line and both B&C on the curve at the same time, the slope between points B&C will equal the Contrast Index. Make a mark on the curve where B&C intersect it and measure the rise over run to get the slope or CI. The rise is the vertical distance (difference in density) from point B to C and the run is the horizontal distance between the two points.

    As an example, assume B plots on the curve at .30 gross density (at Zone II 1/6 exposure) and C plots at 1.40 gross density (at Zone VII 1/2), and that fbf is 0.15 density. The rise is then C-B or 1.40 - 0.30 = 1.10 density units. The curve will be such that the horizontal distance between B&C is about 1.15 density units (remember the horizontal scale is based on the 0.30 unit interval on the vertical axis). The Contrast Index for this curve would therefore be 1.10/1.15 = .97 approximately.

    If instead the numbers were B = 0.25 density (at Zone I 5/6) and C = 1.05 (at Zone VIII) with fbf = 0.15 this curve would have a CI of (0.1.05-0.25)/6 1/6 exposure zones = (0.1.05-0.25)/1.85 = .43 as shown in the example picture which will hopefully post below.

    It's pretty easy to actually draw this on graph paper and determine CI. It's probably much harder to explain or read.

    Joe
     

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  12. dancqu

    dancqu Member

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    I would not recommend Davis' book. Sensitometry is,
    as has been mentioned in other posts this thread, likely
    incomprehensibly presented. I've not read the book but have
    studied a few of his articles. I've eight volumes of D-Max,
    a quarterly put out by The View Camera Store.

    I've a densitometer and off and on do speed tests, and
    zone density measurements which can be read and/or
    plotted. It's an eight hour a day five days a week
    job for some. Dan
     
  13. Jorge

    Jorge Inactive

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    Uh...let me see if I understand this, on one hand you say:

    I would not recommend Davis' book

    And then you turn around and say:

    I've not read the book but have
    studied a few of his articles


    I really dont mind when people say: "I have read the book and found it useless" but really, saying you have read a few articles and thus do not recommend the book is incredible!
     
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  15. dancqu

    dancqu Member

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    That is not ALL that I said. I also mentioned others this
    thread who have issued guarded warnings and I think them
    credible; I've read some of his work. Those quoted
    portions of my post are all "on one hand". Dan
     
  16. Jorge

    Jorge Inactive

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    What others? I dont see anybody else here saying it is not good. Yes, the BTZS is a dry book but I would think that anybody that is going to recommend it or not, at least knows what they are talking about and has read/used the method.

    I have no personal interest in the BTZS, I dont sell it, I dont give workshops on it, but I know it works and I can recommend it for those starting and wanting to use a closed loop system to better control their materials. The difference is that I have actually read the book, I have used it and have compared it to all the other books I have on the topic such as AA, Fred Picker, Todd and Zakia..etc...

    Your example is like me trying to advice 35 mm shooters and telling them..." you know, I dont have a Canon camera, but I have read the instruction booklet and I think it is not good".....If all you have read about the BTZS is what you read in the DI letters, trust me, you know nothing about the BTZS.
     
  17. sanking

    sanking Member

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    I highly recommend Phil Davis' Beyond the Zone System. It is, without any question, the *most* comprehensible and clearest presentation I have ever read about sensitometry.

    Since you admit that you have not read Beyond the Zone System, my suggestion would be that you get the book and read it before offering any more opinions about it.

    Sandy King
     
  18. dancqu

    dancqu Member

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    Sandy King In Quotes

    "I highly recommend Phil Davis' Beyond the Zone System.
    It is, without any question, the *most* comprehensible and
    clearest presentation I have ever read about sensitometry."

    Having not read the book I can only wonder if sensitometry
    and it's association with exposure and development is it's only
    concern. I've a very vague impression that Beyond the Zone
    System, BTZS, the book, might be called The BTZS System.
    I think the OP this thread, photomc, would like to wade in
    rather than dive in.


    "Since you admit that you have not read Beyond the Zone System,
    my suggestion would be that you get the book and read it before
    offering any more opinions about it."

    I'll not infer any more from my readings of the di newsletter. Dan
     
  19. Ornello

    Ornello Inactive

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    What is it you're trying to do?
     
  20. sanking

    sanking Member

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    Beyond the Zone System contains among other things a very clear and detailed discussion of the Zone System itself. What is not so widely understood is that the data provided by Davis' BTZS system of testing and plotting can be used with either incident metering or with reflective metering using zone system principles. In my opinion the incident system is a bit more precise but the reflective metering with a spot meter may be the best system for certain kinds of subjects.

    I don't claim that Beyond the Zone System is an easy ready. In fact, it takes a lot of concentration to absorb the material. Someone once jokingly remarked to me that Davis should have put some erotica in there every five or six pages as in incentive to keep the reader moving ahead.

    There may be easier systems for learning exposure/development/printing controls but IMO there is none more precise and comprhensive than BTZS.

    Sandy King
     
  21. mikepry

    mikepry Subscriber

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    We are working with very precise and exact materials with photography, even scientific materials if you will. So, doesn't it stand to reason one must approach this in a technical and scientific way if one wants to get the most out of their materials? If not, then go to Walgreens or whatever. It never ceases to amaze me how one can come all the way to working with LF and then stop so short of reaching their full potential in working with their materials. I am also amazed at how when BTZS is mentioned the reactions it seems to bring out. Yeah it's very dry....yeah it is boring....yeah watching the Waltons pit cherries is more exciting, but when the smoke clears it gives us the photographer a boatload of information that will enable us (if applied) to make the best exposures we ever have made. Why then, do people stop short at this point? We've bought big cameras, built darkrooms, bought big expensive film, big expensive lenses and then put the brakes on when we hear BTZS?

    The unique situation with photography is the melding of technical and artistic. There isn't any way around it folks, photography does involve things of a technical nature and one does their craft a huge disservice by not going the distance with the technical. It's really wierd but when one wades through the very easy testing and gets the numbers down it goes quickly into the background and then the artistic takes front seat. It is a challenge to learn but it is learnable. I am not one that is at all accademic and learning this type of thing is hard, very hard for me. I won't be able to sit down and talk heavy numbers with Sandy and Jorge and some of the others but I can go out and make exposures and now print the best prints I have ever been able to make and in far less time. And another thing that fascinates me is the site for BTZS ....no one hardly ever is on the forum. Weeks go by between posts! Why do you suppose this is? It's too dry? Too much work? Too technical? No, it's because once people get through the testing they are out making photographs and no real need for stuff like....."What's a good film"? or "Got my first batch of AZO today and can't wait to try it out" or "What's a good paper to print crappy pictures of my kids with?" or "What's better...PMK, PYrocat or ABC?" and one of my all time favorites..."What's a good fixer?" What's a good fixer???????? Paaaleeezzzz.
     
  22. Donald Miller

    Donald Miller Member

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    Mike...very well stated...and I concur completely...But then look at how boring this site would be if we didn't discuss all of the assorted and sundry reasons why we can't get shadow detail in our prints, or why we can't get photographs that exhibit any meaningful tonal range, or what developer is the latest and greatest thing on the block, what paper is the greatest thing since sliced bread, or why Bergger BPF doesn't live up to it's advertised claims, or why we need to burn and dodge to distraction, or why my dog hates me, or my wife won't talk to me any longer. Hell, I would miss all the entertainment if everyone suddenly did the work to find the answers for themselves.
     
  23. sanking

    sanking Member

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    BTW, why is this discussion in the Alternative Photography forum?

    Should it not be under B&W or Exposure?

    Sandy
     
  24. photomc

    photomc Member

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    Sandy, may not have been the best place for this...but I ask here since the prints from the negatives are intended for alternative process (Ziatypes,
    plt/pld prints). Sean if you feel it needs to be moved, by all means feel free to move it.

    Mike, what a great response...really never know quite how to respond to the 'reactive' responses to zone system or BTZS questions but feel quite like you that the whole thing is more scientific than not.

    The purpose of the post is two fold - first wanted to understand what all those numbers/abbreviations are that I see discussed here - you know the 1.30 vs 0.12 and CI vs DR vs etc. In other words, my intent is to understand that if I used film X, exposed at Y and develop using developer A then I can expect the negative to look like P...which might be the kind of negative that I would use to print a Ziatype, on the other hand if the intended print is to be made on VC silver paper, I might want to make an adjustment to exposure or development or some other variable.

    If this seems like a lot of trouble to some, and it may be - but I ENJOY THIS PART OF THE PROCESS, just as much. If it's not your thing that is OK, just skip over these types of post. Since I have not found what works for me (or maybe I haven't realized it yet), asking those members here that are willing to share what they know is useful. Will I chose the zone system, BTZS, a combination of the two, or something else? Heck, I don't know yet..what I intend to use is what I find works for me, something that will help me enjoy making photographs and consistently produce the type of negatives that will allow me to print what ever type I choose.

    Thanks to all that have responded, it's the input from everyone that is important...how we choose to use it is up to us.
     
  25. smieglitz

    smieglitz Member

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    I'm back at work, it is Monday (hooray ???) and I'm able to check these numbers. In the 1987 version of Kodak Professional Black-and-White Films (publication F-5) on page 19 there is a description on how to determine CI. The figures for the CI ruler I quoted last week are in fact correct.

    Now, I'm trying to see where the conversation got sidetracked into one of BTZS rather than CI. As I do, I'm looking at Davis' 1981 version of BTZS and trying to see how CI fits into his system. It would appear to me that Davis is actually defining density range and subject brightness range on the basis of Ilford's G-bar rather than Kodak's CI, and that these two determinants might yield different values for DR and SBR. For example, the graphs on page 35 (under the "Curve Gradient Measurements" heading) would appear to indicate greater SBR and DR when using CI vs G-bar assuming those graphs are scaled the same. Is this correct?

    If so, couldn't I just take all my CI curves and convert them to the simpler G-bar measurement and then determine SBR and DR ala Davis? Then, I'd at least be on the same page as the rest of you perhaps.

    Conversely, I'm wondering if the Kodak system might actually represent the standard way of expressing SBR and DR? Or, has the Ilford/Davis method become the standard amongst photographers? I'm wondering because last year if y'all recall, I was at my wits end trying to determine what various authors meant by the term "density range" in relation to their film exposures needed for alternative process printing in various media.

    Joe
     
  26. Ornello

    Ornello Inactive

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    From what I've read on the subject, the two systems (Kodak and Ilford) are not in complete agreement. Kodak claims that negatives developed to the same CI will print on the same grade of paper, not that they will look the same when so printed. The reference points on the Kodak system are closer together. The high density point is lower in the Kodak system.
     
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