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  1. #61
    Lee L's Avatar
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    You can tell that Mr. Gainer used to design flexible, field-usable aids to navigation and other such things professionally. The attachment is a .pdf file that will make a little card to carry for reciprocity adjustments using Gainer's numbers. (I did a .pdf to make sure it holds formatting and can be printed and resized properly.) The first two columns are metered exposure times expressed as minutes in the first column or as seconds in the second column. These "just happen" to match the time markings on my light meter. The third column shows basic correction times in seconds, calculated by raising the metered time to the power of 1.62 per Gainer. The right hand box indicates Gainer's factor by which to multiply the "Basic Correction", dependent on which film you're using.

    You get your final exposure time by metering, reading the corresponding basic correction, multiplying the basic correction by the film's Bond-Gainer reciprocity factor, then adding the result to the metered time. It's very simple if you have a calculator at hand and can do multiplication. An example is on the attached .pdf "card" to carry along as an aid in using the card. This is a preliminary stab at this, and I may work on making it more user friendly as time allows.

    Someone with the time and inclination could make a slide rule version of this information for field use, with a chart of film factors or a place to list your own factors on the back. A circular version on card stock could be very packable and handy. Yes, I'm old enough to have learned to use a slide rule, and I still have a couple of them.

    As for Ilford FP4+ and it's relation to FP5+, Reeves's data shows that FP4+ has marginally more reciprocity failure at 128 seconds of exposure, a loss of 1.2 stops as opposed to 1.18 for FP5+. This translates to a Schwarzschild exponent of .79 for FP4+ and .83 for FP5+. That points to Mr. Gainer's comments being right on target regarding exposing FP4+ at the adjustments for FP5+.

    I also believe that Mr. Bond's data is accurate and Mr. Gainer's analysis is correct. I need to order the back issues so I can have a first-hand look and learn more. I have no way to prove this, but I think the manufacturers assume that anyone using their film for critical work will test under conditions of use, and that they don't think that testing under manufacturers' lab conditions will have a lot of bearing in the always unique circumstances in the field. Therefore, they only provide general guidelines that will get the users close enough on the first try to have something usable, and from which to make their own adjustments.

    Lee
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  2. #62
    Bruce Osgood's Avatar
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    Gadget writes:
    "Log(tc) = log(tc,1) + 1.62 log(tm)

    where tc,1 is the correction at 1 second indicated time."

    So, to find out what a films tc,1 is, you must conduct a test, OK, what is this test?

  3. #63

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    The data provides the following numbers (assuming the reasonableness of the given factors).
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  4. #64
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    When I went to calculate the adjusted time for 400TX at 90 seconds after I opened the message box, my chart scrolled off the screen. I just recalculated it, but I used 1.618 as the magic number whereas I had used 1.62 when I calculated the numbers for the chart. I doubt the 2 seconds or so will ever be missed out of 339. I hope I never have to do an indicated 90 second exposure. I get impatient with a 5 minute developing time.
    Gadget Gainer

  5. #65
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    Quote Originally Posted by Bruce (Camclicker)
    Gadget writes:
    "Log(tc) = log(tc,1) + 1.62 log(tm)

    where tc,1 is the correction at 1 second indicated time."

    So, to find out what a films tc,1 is, you must conduct a test, OK, what is this test?
    If you know the correction for ANY indicated time, you can calculate the film factor by reading the basic correction off the curve or from the chart and dividing that number into the amount you had to add to correct for reciprocal trade disagreement. Most film makers give you something, and the values I have seen will more likely give you some over exposure, so you should be able to zero in on a reasonable film factor after 1 or 2 trials.
    Gadget Gainer

  6. #66
    kaiyen's Avatar
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    yay! I can now populate a roughly accurate chart that has multiple films on it! then I can (hopefully) add in proper film factors after I (hopefully) do the appropriate testing.

    I'll add more and more films to it over time, until I have the entire world included! mwaaahahahah...

    g'night.
    allan

  7. #67
    Bob Carnie's Avatar
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    Mr Gainer

    I see where I made the mistake and I now do understand the chart
    Thank you very much for your patience and time . I am starting to do a series of night exposures that I believe this chart will be invaluable for my project.

  8. #68
    Lee L's Avatar
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    A Gainer reciprocity method primer

    I've been at home with sick kids for the last few days while my wife is out of town. Working on this between other things has been much more interesting than cleaning house, and gave me something to ponder when I was cooking and cleaning. I've been aware of the Schwarzschild method for calculating reciprocity law failure for years, and was interested in how Gainer's method may relate to it, especially since Gainer's is based on readily available data that was carefully acquired. Every time I read a manufacturer's reciprocity data I get the feeling that I'm looking at a rough guess with Schwarzschild applied.

    Robert Reeves, as I have mentioned before, did a lot of film tests prior to 2000, and his method and results can be seen at:
    http://www.robertreeves.com/filmtest.htm
    with specific B&W results at:
    http://www.robertreeves.com/b&w.htm
    Look at it when you get a chance. It's worthwhile.

    So I've been trying to find a way to convert Reeves' Schwarzschild exponents to Gainer film factors. I can find no direct mathematical conversion that's convincing. However, I did take the Schwarzschild equations and exponents from Reeves and the Gainer formula and factors posted here (I don't have the article at hand), and I took the four B&W films they share in common as a basis for finding out how closely the methods match. I found that I could apply the same corrections for all four films common to both tests and get calculated exposures from the Gainer formulas and from the Reeves test results (with conversions applied to yield Gainer film factors), that all agreed within about 1/6th of a stop. This agreement is across four films at exposure times of 1 sec, 128 seconds, and 900 seconds of metered exposure, and also across probable emulsion changes. I figured that was close enough to make the conversions useful as a starting point when applied to films Reeves tested that are not in the Bond data.

    So I've attached the results in a three page .pdf file, with a Gainer basic correction chart (log axes with instructions for reading log axes), a film factor table with 13 B&W films (5 from Bond data and 8 from Reeves data), instructions, and a worked example. In case you want to know if this is immediately applicable to films you use, the films added from the Reeves data that aren't in Bond's data are: Pan F (not Plus), FP4+, Plus-X, Tech Pan (bye-bye), Ilford SFX, XP-2, T400CN, and TMZ.

    Lee
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  9. #69
    cao
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    What is the correction preserving?

    Forgive me if I seem dense, but what is the correction preserving? Without even considering the developer, we could think in terms of density of development sites for a given light in a given time. Are we preserving zone V? How is this site density curve being changed? What I am shooting at is this; I have the niggling feeling that the curve is changed in a way that isn't fully correctable by changing development. If this is the case, then we can stretch tones around by deliberatly exposing so as to create RF, and hence RF can be used as a creative control with the right films. I'm seeking a correction function mapping an site density curve to a different curve in the face of RF. Do we already know this from Gainer's correction?

  10. #70
    Lee L's Avatar
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    Quote Originally Posted by cao
    Forgive me if I seem dense, but what is the correction preserving? Without even considering the developer, we could think in terms of density of development sites for a given light in a given time. Are we preserving zone V? How is this site density curve being changed? What I am shooting at is this; I have the niggling feeling that the curve is changed in a way that isn't fully correctable by changing development. If this is the case, then we can stretch tones around by deliberatly exposing so as to create RF, and hence RF can be used as a creative control with the right films. I'm seeking a correction function mapping an site density curve to a different curve in the face of RF. Do we already know this from Gainer's correction?
    This is always part of the problem with reciprocity failure. The Kodak recommendations for development adjustment earlier in this thread are a good place to start, but I don't have the backissue of the Bond article in Photo Techniques, so I can't say what he did. I can tell you that everything I've seen done in astrophotography appears to be aimed at preserving density on a gray card, or Zone V. I expect that Bond has tried to preserve the entire tonal scale to whatever degree is possible, but can't be sure until I see the article. Look at the Reeves websites in my last post to see the standard astrophotographers technique for determining the Schwarzschild exponent. As always, you'll probably have to test materials for your own workflow. Even then, Covington hints that the variation in film batches is enough to swamp your standard adjustment, especially at the typical 8 minute to hours long exposures in film astrophotography. Astrophotographers aren't typically concerned about preserving a specific dynamic range, they mostly want to go as faint as possible without getting the sky background washed out too much, and there are varying atmospheric conditions that determine that limit, often unpredictably. Astrophotographers bracket exposure times as much as possible.

    I know this doesn't help with the moonlit landscape. I need to get the Bond and Gainer articles myself. You're not being dense, it's just that the testing for this is very tedious, and I've never seen anyone do an analysis as complete as the Bond article seems to be. I do recall having read some techniques used by cathedral photographers, including stand development, but that was years ago, and I don't have a ready reference. Read the Reeves web pages to get an idea of how to procede with your own testing using a Wratten #96 or a B+W 110 filter. (If you have and IR sensitive film, the Wratten #96 is not typically a 3.0 density in those wavelengths, so experienced people say to go with the B+W 110.)

    There are a couple of books out there on "night photography" that might have useful information, but I haven't seen them.

    Lee



 

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