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  1. #21
    Rafal Lukawiecki's Avatar
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    Quote Originally Posted by Stephen Benskin View Post
    The testing under the ISO standard and the determination of the ISO triangle is done under no flare conditions. Rafal's test consisted of sticking the step tablet up against the window and shooting it with his camera (optical system). He emphasized the need for testing under daylight conditions but missed that such a set-up will produce flare. As we know flare compresses the shadow area. ISO speed is determined using the gradient of the shadow. CI is determined more from a fuller range of the film. In order to raise the gradient of the compressed shadows sufficient enough to fit into the ISO triangle, the CI will be excessive like we see with Rafal's examples.

    What is happening here is that a non-flare interpretation is being used with a test that incorporated flare. Any results are questionable at best.
    Thanks for pointing this out. On this thread I asked about the correct way to photograph the transmission tablet. It was suggested that I should tape it to a window. In post 7 I was concerned about any reflections from the surface of it, when it is taped to a window.

    Are those reflections the main source of the flare that you are referring to, or could they be an additional reason for the steep curves which resulted in my test?
    Rafal Lukawiecki
    See rafal.net | Read rafal.net/articles

  2. #22

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    Reflections could definitely distort results but I would expect those errors to be more obvious.

    Flare could be coming from several sources in this test setup. First there's the bright area around the transmission tablet. There are several ways of reducing this effect, such as surrounding the tablet with black paper, or masking the lens or masking the camera or combinations of these.

    Then there's the fact using a backlit transmission tablet (or a front-lit reflection tablet) will put some degree of flare into the test anyway. As long as you are using a wedge/tablet (ie a wide range of adjacent "luminance values" in a single frame), there is no way to eliminate that kind of veiling flare other than contacting the wedge, which is the no-flare method Stephen refers to.

    The problem is to balance the factors and make sure the test is meaningful. For example, in order to generate the "purest" possible (let's call it "native") comparitive curves of different films in some sort of standardized way, contacting is useful because it removes flare from the tests. However, to then evaluate a film in the context of the actual photography you will be doing, it always seems to me contacting is too far removed from actual conditions to give you an accurate picture of what the film is doing - unless you typically photograph under low-flare conditions. Depending on flare, speed can change, as can contrast particularly in the low densities (ie shadow values). Of course speed and toe contrast are interrelated. Flare flattens local contrast in the low densities.

    I keep meaning to post the curve results of a comparison (flare/no flare) test to the other thread I started recently regarding flare, but I just haven't had the chance. Hopefully this weekend.

  3. #23
    Stephen Benskin's Avatar
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    [QUOTE=Michael R 1974;1386719]
    However, to then evaluate a film in the context of the actual photography you will be doing, it always seems to me contacting is too far removed from actual conditions to give you an accurate picture of what the film is doing - unless you typically photograph under low-flare conditions. Depending on flare, speed can change, as can contrast particularly in the low densities (ie shadow values). Of course speed and toe contrast are interrelated. Flare flattens local contrast in the low densities.
    QUOTE]

    This is where the Rafal's test is such a good example. All of the methods used to evalute the film curve are based on a no flare curve but are related to how the film will respond in use. ISO film speed assumes a touch over a stop flare factor. The fractional gradient method's shadow gradient concerns itself with the gradient of the contacted film test but it translates to what to the shadow gradient is when the film is shot under average flare conditions. When calculating the CI to process the film to, you simply take the log subject luminance range and subtract the flare before dividing it into the paper LER aim. If you have a curve that incorporates flare, the various testing methods will not produce accurate results.

  4. #24

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    Stephen, this is still confusing to me. Are you saying the tests are conducted under no-flare conditions (ie contacting) and that flare impacts are then manually overlayed on top of the raw no-flare curves to give a working curve for the determination of speed, development time etc? If so, how exactly is this done? How, for example, does one add a one stop flare factor to a given zero flare curve?

  5. #25
    Stephen Benskin's Avatar
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    Quote Originally Posted by Michael R 1974 View Post
    Stephen, this is still confusing to me. Are you saying the tests are conducted under no-flare conditions (ie contacting) and that flare impacts are then manually overlayed on top of the raw no-flare curves to give a working curve for the determination of speed, development time etc? If so, how exactly is this done? How, for example, does one add a one stop flare factor to a given zero flare curve?
    Michael, there are two different ways to explain how this is possible with the ISO standard. The standard's contrast parameters (ISO Triangle) is really part of an equation that creates a good correlation between the fixed density method and the fractional gradient method. If a correlation didn't exist, I doubt the fixed density method would have been adopted. What this means is while the speed point is at a density of 0.10 over Fb+f, when the parameters are met, the factional gradient speed point will always fall approximately one stop to the right. As we all know, the fractional gradient point is the minimum point of exposure where a print considered of excellent quality can be produced from the negative.

    This except from Safety Factors in Camera Exposure by C.N. Nelson explains the reasoning.

    "Proposed Change in Speed Criterion

    The reduction in the safety factor could be accomplished simply by changing the constant in the ASA formula for deriving the ASA exposure index from the ASA fractional-gradient speed of the film. The present formula, which gives a safety factor of about 2.4, is

    Exposure Index = Fractional-Gradient Speed / 4Es

    or Exposure Index = 1/ 4Es

    where Es is the exposure in meter-candle-seconds at the fractional-gradient speed point and 1/Es is the ASA fractional-gradient speed. If the constant of ¼ were replaced by a constant of ½, a new type of "exposure index" would be obtained which would provide the proposed lower safety factor of about 1.2.
    There are several reasons, however, for adopting not only a new constant but also a different speed criterion. The fractional-gradient criterion was originally chosen because it has the desirable feature of giving speeds that correlate closely with speeds obtained by practical picture tests. It has the objectionable feature, however, of being somewhat inconvenient and difficult to use. Consequently, a simpler and more convenient criterion, such as that based on a fixed density above fog density, is often desired. Fortunately, as shown by the recent data of Nelson and Simonds, "a good correlation exists between fractional-gradient speed and speeds based on a density of 0.1 above fog, provided the development condition's are controlled so that a fixed "average gradient" is obtained...Thus the adoption of the 0.1 fixed-density speed criterion in combination with a suitable development specification would offer the advantages of convenience and practical significance."

    The other way to explain it is easier to picture. The ratio between the 0.10 fixed speed point and the metered exposure point is 10x or a range of 1.0 log-H. That's 3 1/3 stops. The average shadow falls 4 1/3 stops below the metered exposure point. That's a difference of a stop. The no flare curve uses a point one stop above where the shadows fall as the speed point because in practice flare will bring those shadows up.

    The reason why Zone System tested EIs tend to be 1/2 to 1 stop lower than the ISO speeds comes from a misinterpretation of the above concept. As I've pointed out before, in camera testing of a single toned target will yield almost zero flare. Zone System speed point falls 4 stops down from the meter exposure point or 2/3 stops below the ISO speed point with the 10x ratio. The Zone System testing method assumes a flare test condition for a no flare test condition thus causing the speed discrepancy between ISO film speeds and Zone System EIs.
    Last edited by Stephen Benskin; 08-29-2012 at 07:04 PM. Click to view previous post history.

  6. #26
    Rafal Lukawiecki's Avatar
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    Stephen, when you write that Zone System speed point falls 2/3 stops below the ISO speedpoint, do you mean it is below 0.1 fb+f? Also, I wonder if the WBM choice of 0.17 as the speedpoint means that its CIs are, in essence, no longer comparable to ISO CIs.

    On another note, I am finding all the contributions on this thread very informative, thank you everyone. I am not sure, however, as to the best way to interpret my results to find the development times. I realise that one can take great pictures without any of this, but I would love to know my personal timings for N-1, N, and N+1. Perhaps I need to contact the tablet for the curves, and to perform a separate EI test in natural light using a non-uniform target and my camera.
    Rafal Lukawiecki
    See rafal.net | Read rafal.net/articles

  7. #27
    Stephen Benskin's Avatar
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    Quote Originally Posted by Rafal Lukawiecki View Post
    Stephen, when you write that Zone System speed point falls 2/3 stops below the ISO speedpoint, do you mean it is below 0.1 fb+f?
    No, it's about the misinterpretation of the ratio between the speed point and the metered exposure point caused by combining a flare interpretation with effectively a no flare testing situation. While the shadow exposure for an average scene does fall around 4 stops down from the metered exposure point (4 1/3 to be exact), the speed point doesn't. If you stop down four stops below the metered exposure point, the exposure will fall 2/3 of a stop below 0.10. In order to bring it up to 0.10, the exposure needs to be increased by 2/3 of a stop which accounts for the lower EIs encountered in Zone System testing. I did a thread on the ratio sometime ago.

    Click image for larger version. 

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    On another note, I am finding all the contributions on this thread very informative, thank you everyone. I am not sure, however, as to the best way to interpret my results to find the development times. I realise that one can take great pictures without any of this, but I would love to know my personal timings for N-1, N, and N+1. Perhaps I need to contact the tablet for the curves, and to perform a separate EI test in natural light using a non-uniform target and my camera.
    I believe it's very important to understand the theory. The solution to your problem is retesting, and you shouldn't be so concerned about the film speed. As to exactly what N, N+1, etc should be in terms of aim CIs and why is another interesting point of theory.
    Last edited by Stephen Benskin; 08-30-2012 at 01:12 AM. Click to view previous post history.

  8. #28

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    Stephen, I'm trying to come up with a decent illustrative test example to show flare vs no flare. For the flare test I was going to photograph a reflection tablet. Here's a question a I have (actually this would apply to transmission wedges too).

    Most tablets have a progressive set of densities. Therefore the difference in density between any two adjacent steps is relatively small. We know that flare affects the highest reflection densities on the reflection tablet most. Consider the step with the highest reflection density. Suppose we placed the step with the lowest reflection density right next to it, rather than further away. Does that change the amount of flare impacting the step with the high reflection density?

    Asked another way, if one fills the frame with a front lit reflection tablet or backlit transmission wedge of a given total density range, does the arrangement of the steps on the tablet/wedge have an effect on the way flare affects the resulting curve? Or is it only the total range that matters?

    As an aside, why are most reflection tablets glossy? It would be easier to work with them if they were not.

  9. #29
    Stephen Benskin's Avatar
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    Quote Originally Posted by Michael R 1974 View Post
    Stephen, I'm trying to come up with a decent illustrative test example to show flare vs no flare. For the flare test I was going to photograph a reflection tablet. Here's a question a I have (actually this would apply to transmission wedges too).

    Most tablets have a progressive set of densities. Therefore the difference in density between any two adjacent steps is relatively small. We know that flare affects the highest reflection densities on the reflection tablet most. Consider the step with the highest reflection density. Suppose we placed the step with the lowest reflection density right next to it, rather than further away. Does that change the amount of flare impacting the step with the high reflection density?

    Asked another way, if one fills the frame with a front lit reflection tablet or backlit transmission wedge of a given total density range, does the arrangement of the steps on the tablet/wedge have an effect on the way flare affects the resulting curve? Or is it only the total range that matters?

    As an aside, why are most reflection tablets glossy? It would be easier to work with them if they were not.
    Distribution does play a factor. We've all seen the bleed from a really light area next to a really dark area. I guess this could be consider a localized flare effect in addition to the veiling flare.

    The reason why reflection tablets are glossy is that glossy produces a greater reflection density range. Think about the difference between the D-Max of matte paper at around 1.60 with glossy paper at around 2.00 to 2.20. I know what you're saying about the difficulty in handling. It's kind of ironic that reflectance is based on a Lambertain surface.
    Last edited by Stephen Benskin; 08-31-2012 at 12:34 AM. Click to view previous post history.

  10. #30

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    Ok, so I think my test will still be at least somewhat effective in showing the effect of veiling flare even though it might not show the localized flare effect (perhaps I could try that in a third test).

    It of course makes sense the density range of the tablet will be higher on a glossy surface, but for film testing personally I think we could do with the reduced range of a matte finish. You can't plot a full range film curve from a single exposure of a step tablet anyway, so I'd rather have to do an extra exposure and not have to deal with reflection problems. You have to be careful how you orient the tablet and meter to avoid reflections ruining the test. It is an annoyance.

    Apologies to Rafal for hijacking this. I wasn't sure if I should post my questions to this thread or the one I already had going in case nobody was reading it.



 

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