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  1. #11
    Stephen Benskin's Avatar
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    Okay, what about the very concept of average gradient? How many threads are about whether a film is more or less contrasty in developer A or developer B?

    If everyone used average gradient, this wouldn't be a question. As long as the film's average gradients match, they have the same contrast and will produce identical prints (excluding local contrast differences). If people did a simple test that defines the film contrast, a huge amount of the threads here wouldn't exist, as well as a few photographic urban myths. Say good-bye to the question of films being inherently contrasty, say so long to magic developers, and say arrivederci to vague music analogizes to explain what to expect from a film/developer combination.

    I have a saying, "contrast is contrast." While the saying may not be sweeping the country, it is never-the-less true. If properly determined, a gradient of 0.58 means the same thing no matter the film type.
    Last edited by Stephen Benskin; 09-18-2011 at 07:34 PM. Click to view previous post history.

  2. #12
    Bill Burk's Avatar
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    Quote Originally Posted by Stephen Benskin View Post
    .... As this is done with processing, average gradient is used in the darkroom. However, you still decide how the film is going to be processed in the field, N, N+1, etc. It's when you get back to the darkroom that N becomes CI 0.58 and according to the Time/CI curve example for Tri-X in Xtol that means a processing time of 6:15.....
    What is funny is I was about to say. But I am not choosing 0.58 for N because I am choosing 1.00 LER.

    I am also choosing 7 stops SBR.

    Wait, it's 0.59

    Again, arriving at the same number but for different reasons.

  3. #13

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    Stephen, contrast is not contrast. For people like me who are often photographing under extreme luminance range conditions, average gradient methods are on oversimplification I'm afraid. I need to know what the film's curve inherently tends to from threshold to 15 or 16 stops above that. Different films have different curve shapes over a range that long. Some films are indeed "contrastier" in the sense they don't have as long a scale. Different films respond differently to different developers when it comes to the shape of the curve. And I'm not talking about esoteric intricacies that don't manifest themselves in the real world. These are visible differences.

  4. #14
    eclarke's Avatar
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    God, I'm not scientific enough..I look at the print, decide how I want to change it and make another until I like it (usually 3 or 4 sheets)...EC

  5. #15

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    Quote Originally Posted by eclarke View Post
    God, I'm not scientific enough..I look at the print, decide how I want to change it and make another until I like it (usually 3 or 4 sheets)...EC
    I totally agree at the printing stage (although I usually use a hell of a lot more paper than 3 or 4 sheets). When I make negatives I'm concerned with retaining all the information I can, with good separations over as much of the curve as possible. I don't "target" a paper grade or anything like that. And when I'm printing it's all by eye. I start with a soft grade and work my way up, with whatever amount of burning, dodging and other acrobatics are necessary. There are no printing "formulas" for me.

  6. #16
    ffg
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    Michael: This is what my discussion with Stephen was all about. The thing is that if you evaluate your average gradient with respect to those "extreme" points of the film curve you reach the same conclusion as looking at the curve itself. That is excluding local contrast. However you get in trouble when you try to generalize the analysis of normal situation to extreme situations, and this is the reason I don't like to use average gradients at all (here I mean the average slope of the function over a certain region.)

  7. #17

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    Agreed. Don't get me wrong I'm not saying Stephen is incorrect. We've exchanged notes in the past on these subjects. I always find his threads interesting and I enjoy learning not only about the gradient methods, but also about the potential pitfalls in testing and interpreting our results.

    However in the end, for the reasons you've suggested, I don't use gradient methods. I test and plot. I find it more straight forward and readily revealing, particularly when comparing films and/or developers, and the same basic methodology can be applied to extreme situations. Local contrast at various locations on the curve is extremely important to me, as is the location and shape of the shoulder.

    As for BTZS, I've never personally thought it adds much value.

  8. #18
    Stephen Benskin's Avatar
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    Questioning the range of accuracy of a method is what this thread is about. Does the specific gradient method include the entire area of the curve that will be used in the field while excluding the areas of the curve that aren’t? This generally means matching the log-H range of the curve to the luminance range of the scene photographed.

    I just don’t think it’s fair to use extreme examples for which the method isn’t even designed. Most of the situations photographers run into fall within a rather narrow range (see the Normal distribution Curve). Ninety-five percent all scenes fall within a 2 1/2 stop range of the average luminance or in other words from -2 ½ to + 2 ½. Sixty-eight percent fall within a 1 ¼ stop range of average luminance. It seems to me that the broader the test parameters, the higher the rate of potential error. It makes sense that any method would want to restrict the testing parameters as much as realistically possible. This means keeping them as close to the type of situations encountered most.

    Aside from the way Davis decided to compensate for flare in the test, I believe his method is perhaps the best for the individual. He uses the NDR to define the height of the triangle (rise), and the log-H range for the base (run). It maybe best for the individual, but not in general.

    A major purpose of any method is communication, and that includes the method in which the test was made. Placing “ISO” in front a film speed means that the standard’s very specific procedures have been followed. Anyone familiar with the standard understands how the film was tested. As is the same with the various average gradient methods.

    This is the main reason why the Davis method works extremely well for individual use but not as well for general usage. For that to happen, there would have to be average gradient values for each potential value of the NDR for each curve. A good average gradient method needs to reflect specific shooting conditions yet not be too specific to be relevant for the gradient value to be used as a general reference.

    I believe Contrast Index does that. Davis mischaracterizes CI as always measuring “a fixed portion of curve length, their values are unrelated to subject range.” The reality is quite the opposite. The key is the arc. As the “Contrast Index” paper states, “ as the log exposure range of the normally used part of the curve decreases, the average gradient, density range, and minimum and maximum densities increase.” In other words, as the arc increases, it covers a progressively smaller log-H range, so it tends to measure the part of the film curve the would normally be involved in the exposure at a given subject luminance range.

    Now, average gradient isn’t a panacea and using it doesn’t mean everything is going to print effort free. But it will get you close enough the first time out. The graph example shows TX and HP5P both developed to a CI 0.61. I didn’t have to look hard for the two film examples either. I just went through my database looking for two films with the same CI. It appears that a simple numerical value can describe a curve quite well for the average conditions expected to be covered.

    What about those situations that fall outside the intended parameters? What do you do for those really long luminance ranges? Just change the parameters. With something like CI, use a bigger arc. But for the really extreme situations, the Davis NDR version can be very effective. The graph attachment with the extended curve shows how this works. In Curve A the 1.05 NDR intersect the curve at Δ 2.98 log-H (10 Stops) for an average G of 0.35. Curve B extension illustrates a slight shouldering off. With this curve 1.05 NDR doesn’t intersect the curve until Δ3.75 log-H (12.5 stops) for an average G of 0.29.

    Check it out and see how closely the numbers work out. Pick a LSLR and plug it into this equation:

    CI * LSLR = NDR

    See how closely the NDR on the curve matches the projected NDR from the equation.

    Film is the intermediate step between the contrast of the subject and placing it onto the contrast of the paper. Processing the film can be thought of as a black box approach. We know what goes into the camera (luminance range + flare) and we know what comes out of the processed negative (density). By comparing the output to the input, we get a picture of what is going on with the processing.

    Whether you use a quantifiable approach with step tablets and curves or a more instinctive hit and miss approach, the objective is the same. The goal is to define what the processing has done to the film in order to have greater control over the materials used in our art.
    Attached Thumbnails Attached Thumbnails Normal Distribution Curve.jpg   TX 0.61.jpg   delta 0.35.jpg  

  9. #19
    Bill Burk's Avatar
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    A point I think Michael R 1974 is making is that some photographers seek out the far-beyond-average scene. For example looking out from the interior of a beach cave. A point you made in an earlier post Stephen, that if you use a highly active developer the toe doesn't get a chance to "come up to speed". These two examples are not well served by CI and are scenarios where the shape of the curve is significant to the results.

    The straight photography on graded silver gelatin paper that I do is well-served by the CI model, it is great to see that less exposure range is used as less is relevant to the final print. And I just thought it was a convenient shape for the graph. Well, I knew the arc measured less exposure range as the CI went up, just didn't know it was deliberate.

  10. #20
    Stephen Benskin's Avatar
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    Bill, it was the whole concept of average gradient that was in question. I believe I showed how those can be handled. It's all about contrast. But if not an average contrast method of some kind than what other option is there to use? If not, then what?

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