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Thread: Ilford - new reciprocity failure compensation factors

  1. #21
    ic-racer's Avatar
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    Re: Ilford - new reciprocity failure compensation factors

    I like it when they keep this stuff a secret. The easier it is to do low-light photography, the less "wow" and "how did he do that" factor there is in Large Format low light photography.

  2. #22
    Randy Moe's Avatar
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    Re: Ilford - new reciprocity failure compensation factors

    Quote Originally Posted by Drew Wiley View Post
    Do they state what their specific light source was (K)? Even if threshold exposures are matched, significantly different colors of filtration cannot just be corrected with a filter factor, because the contrast gamma also significantly diverges at long exposures, and is inconsistent from one film to another, and even with the same film at different lengths of exposure. You're back to square one, realistically needing to test in advance with analogous parameters.
    You could click on the Ilford link...

  3. #23

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    Re: Ilford - new reciprocity failure compensation factors

    Quote Originally Posted by Pere Casals View Post
    Yes... I shot at night with HP5

    David, the newly Ilford published correction factors are accurate, but only useful for very careful spot metering en each zone. The former graph in datasheets contains a safety factor of aprox 2/3 stop (for 15s base at least), and it is a good advice for an starting point.

    The right correction factors are very sensitive to the way we meter, and to the other factors, so IMHO the old good graph recommends a good starting point.

    For example, the graph says that 15s base has to be corrected to 50s, while the new factors say 35s for HP5 and 30s for FP4.

    Principal problem with correction factors is that the shadows have more failure than mids and highlights, so one thing is correcting for the scene average illumination and another one is correcting for the shadows.

    IMHO if we have a low contrast dull scene the new factors can be used, but with a contrasty scene (urban night) we should apply an additional correction if we want some shadow detail, and in this case the old graph included a lot of wisdom to get our starting point.


    Lower correction factors were published by Howard Bond 5 years ago: http://phototechmag.com/kodak-ilford...te-films-2013/

    One has to be careful when comparing LIRF features of different film manufacturers, because ilford used a generous safety margin for good practical results.

    Anyway, if one want to do the thng perfect one should calibrate LIRF, this is making contact prints of the Stouffer wedge with known lux, and using also long exposures (15, 30, 60s , 120 for example) and different N+/-. With that information one can guess very well what density he will obtain in the negative for each scene metered spot. I made some informal tests in this way, enough to see that full calibration it's interesting for night shots.


    Let me repeat, IMHO new factors are useful for accurate metering, if one wants to use spot metering in the shadows to know in what density it will end a "zone" in the negative, but better make calibrations with the Stouffer. If it's not the case, better to start with the old graph, because that safety margin is benefical to conserve shadows.
    Pere,

    Ilford used a fixed density criterion (0.1 above Fb+fog) to determine these t-factors. As you know, the speed point under ISO conditions is the exposure Hm where density is 0.1 above Fb+fog. Using this as a reference point, they then plotted characteristic curves for successively longer exposures and measured the speed losses relative to Hm, by finding the exposure in each case that produced a density of 0.1 above Fb+fog.

    Said another way, the equation gives you the adjusted time so that the density 3 1/3 stops below the metered exposure is always 0.1.

    From a Zone System perspective (for those who use it), the equation can be said to give you a constant density at Zone I 2/3.

    This is a little more conservative than Howard Bond's tests which targeted a constant density at Zone III.

    Hope this helps.

  4. #24

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    Re: Ilford - new reciprocity failure compensation factors

    Quote Originally Posted by Michael R View Post
    Pere,

    Ilford used a fixed density criterion (0.1 above FB+fog) to determine these t-factors.
    Hello Michael,

    The ilford technical notice https://www.ilfordphoto.com/wp/wp-co...mpensation.pdf speaks about "effective speed", not about ISO speed.

    I understand what you say, giving a correction factor for the speed point it is not the same than for the meter point that has exactly x10 (3 1/3 stops) more light intensity...

    IMHO a key factor in this discussion is if the new ilford correction is for the speed point or not. If it was for the speed point IMHO new factors would be higher than the old curve (I guess), but it's the counter.

    Still, do you know for sure that the new values are based in the fixed density criterion (0.1 above FB+fog) ? Or factors are for "the effective speed" ? What is the "effective speed" ?

    Regards

  5. #25

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    Re: Ilford - new reciprocity failure compensation factors

    ISO speed point is the reference point, and the t-factors are based on the loss of effective speed relative to the ISO speed point.

    Effective speed is based on finding the exposure that produces a density of 0.1 above Fb+fog under LIRF conditions. It is called effective speed because it is based on a fixed density criterion (ISO contrast conditions do not apply under LIRF conditions due to the inherent contrast increase).

    Another way of saying this is that the correction is for the speed point (using your words).

    This is the most logical way of doing it. It isn't very complicated, just time consuming to test without the right equipment.

    The old curves were generic and also were not film-specific.

  6. #26
    Randy Moe's Avatar
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    Re: Ilford - new reciprocity failure compensation factors

    When you guys get done, will you please give a summation and succinct advice on actually using Ilford's new guideline.

    Must be comprehensible to a 5th-grade level.

    And very short.

    Carry on, we can wait.

  7. #27

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    Re: Ilford - new reciprocity failure compensation factors

    1) Look up the P factor for the film you are using in the Ilford table

    2) Whatever your exposure time is (in seconds), put that number to the power of the P factor (the P factor is the exponent), and this is your new exposure time.

    Example - you're using Delta 100 and your meter tells you to expose for 10 seconds:

    1) Use a P factor of 1.26 from the Ilford table

    2) Metered time(P factor)=Adjusted time so 101.26=18.2 (round to 18 seconds)

  8. #28
    Randy Moe's Avatar
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    Re: Ilford - new reciprocity failure compensation factors

    Quote Originally Posted by Michael R View Post
    1) Look up the P factor for the film you are using in the Ilford table

    2) Whatever your exposure time is (in seconds), put that number to the power of the P factor (the P factor is the exponent), and this is your new exposure time.

    Example - you're using Delta 100 and your meter tells you to expose for 10 seconds:

    1) Use a P factor of 1.26 from the Ilford table

    2) Metered time(P factor)=Adjusted time so 101.26=18.2 (round to 18 seconds)
    Not bad, that's exactly what I wrote.

  9. #29

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    Re: Ilford - new reciprocity failure compensation factors

    Quote Originally Posted by Michael R View Post
    ISO speed point is the reference point, and the t-factors are based on the loss of effective speed relative to the ISO speed point.

    Effective speed is based on finding the exposure that produces a density of 0.1 above Fb+fog under LIRF conditions. It is called effective speed because it is based on a fixed density criterion (ISO contrast conditions do not apply under LIRF conditions due to the inherent contrast increase).

    Another way of saying this is that the correction is for the speed point (using your words).

    This is the most logical way of doing it. It isn't very complicated, just time consuming to test without the right equipment.

    The old curves were generic and also were not film-specific.
    Thanks for the explanation

  10. #30
    Drew Wiley
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    Re: Ilford - new reciprocity failure compensation factors

    Still would be more intuitive in graph form. Most of us carry a limited number of films at any given time, maybe only one. Alas, I'm down to my last three sheets of 8x10 Acros.

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