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  1. #81
    gainer's Avatar
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    The true beauty of this serendipitous discovery is that all B&W films seem to share the same basic variation with indicated time, differing only in one constant for each different film. This is still empirical, though someone may have a theory about it. But it certainly saves a lot of experimental drudgery when you don't know how to correct a new film. One exposure series based on the same indicated exposure time can pretty well nail down the film factor at that indicated time. and one more calculated exposure for a different long indicated exposure can tell you if the method indeed does work for that film. If it does, then you've got it made.

    When I began as an aeronautical research engineer at NACA I learned the value of the different kinds of graph paper when you don't have a theory or the theory doesn't work. If it's not linear, try semi-log. If that doesn't work, try log-log.
    Gadget Gainer

  2. #82
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    Reciprocity failure as "expansion"

    Here's my thinking on this, which jibes with other things I've read in the past, and which I _have not tested_ in the real world. It agrees in principle with the information from Covington and others that I've read, and in general with the adjusments in development suggested by Kodak and others. Ryuji Suzuki indicates increased contrast as well at: http://silvergrain.org/Photo-Tech/reciprocity.html which is worthwhile reading if you're concerned about reciprocity failure. He specifically addresses the problem of increased contrast, and uses a polynomial expression for the Covington/Schwarzschild exponent, but doesn't say exactly what it is. If I'm reading it correctly, it might be a third order polynomial.

    So here was my thought process:

    Say you have a scene which has a medium gray and important detail at -2 stops and + 2 stops from the gray card (or incident reading). Without accounting for reciprocity they meter at 64 seconds, 16 seconds, and 4 seconds. With Ilford's recommended 0.51 film factor, that converts to 490 seconds, 61 seconds, and 8.8 seconds using the Gainer formula. The ratios are no longer 0.25/1/4, but become .125/1/6.95 after correction, so that an exposure at the corrected medium gray exposure of 61 seconds is underexposed below that point on the "tonal scale" and overexposed above it. In this instance, 2 stops under becomes 3 stops under and 2 stops over becomes 2.8 stops over. So in Zone system terms, Zone III has dropped to Zone II and Zone VII has risen almost a full stop to Zone VIII. This is caused by doing the exposure compensation for medium gray, which undercompensates for darker areas and overcompensates in lighter areas. So reciprocity itself is introducing a form of "expansion", and reduced development can, to some degree, be used to "contract" that expansion back into a printable range.

    I have done a little spreadsheet that's does this calculation for me, and I can plug in a film factor and three different exposure times to see what happens to the range from -4 to + 5 stops from medium gray in 1 stop steps. It plots a little graph with a line for each of the exposure times, showing how the range of tones changes with extended exposures. Interestingly enough, if you plug in the numbers for TMAX and look at the results relative to the information posted here for development changes with extended exposures by Will S, you get a reasonable match.
    These calculations use the medium gray exposure time for the reciprocity corrections using the Gainer formula, but other values away from medium gray could be used as well. The included chart allows you to view the effect on exposure areas at -4 to +5 stops from medium gray. The changes are calculated by adjusting the exposure for medium gray and comparing the required adjustments for other tones at 1 stop intervals, then taking the ratio between the adjustments required for other tones and the adjusted gray tone time. That ratio is converted to stops and plotted. The black "standard" line is the standard 1:2 ratio per stop to be used as a baseline for comparison. The attached chart uses TMX and the Gainer factor of 0.069 determined from the Bond article.

    This is at least a place to start. I have a nagging feeling that I'm not completely taking care of the compounded effects with this method, so I have to give it some more thought. I'm posting it as a way to stimulate thoughts and comments, not as a final answer to the question.

    Lee
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  3. #83

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    Quote Originally Posted by gainer
    When I began as an aeronautical research engineer at NACA I learned the value of the different kinds of graph paper when you don't have a theory or the theory doesn't work. If it's not linear, try semi-log. If that doesn't work, try log-log.
    I had a friend point out in college that even the crappiest of data sets can sometimes look great when plotted on log-log paper. It's true!

    Kirk - www.keyesphoto.com

  4. #84
    gainer's Avatar
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    Lee, Howard did not find any significant increase in contrast.
    This is a quote from Howard Bond's article in Photo Techniques Magazine.

    "In the past, films typically yielded increased density ranges with long exposures. The extra exposure that would render Zone III as planned was less needed in the high zones, so they were elevated, increasing the density ranges of negatives. This situation is now much improved. At 240 seconds indicated, T-Max 400 and 100 Delta showed no elevation of Zone VIII. Tri-X was up slightly, but the elevation was only slightly more than the typical variation from one attempt to another. Zone VIII was up about 2/3 zone with HP-5+ and T-Max 100. Stating an elevation in terms of zones is very approximate, since the width of a zone (expressed as a range of negative densities) varies greatly with development."

    As you see, the effect of long exposures on the density range from Zone III to Zone VIII is much less than on the the exposure time to achieve a Zone III density with normal development.
    Gadget Gainer

  5. #85

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    I did some film testing this last summer, and I believe I saw this effect of increased contrast with 4 films at 240 seconds - BFP 200, FP4+, 320TXT, and 100TMX - the current version of each of those films. The lower zones were significantly lower in density as I had expected from my understanding of reciprocity effects.

    I also ran Fuji Acros 100 and I really had to look hard to visually tell which film was at 1 sec. and which was at 240. Glad I creased the corner of one of those two films.

    If you are doing long exposure B&W, I really suggest that you try some Acros.

    Of course, since I haven't had time to do much data reduction on these tests, I really have to point out that this is anectodal evidence on my part.

    Kirk - www.keyesphoto.com

  6. #86
    Lee L's Avatar
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    My "reciprocity expansion" spreadsheet is something I built not necessarily to perfectly reflect the real world, but more as a model with which I could "test" the effects of the reciprocity characteristics of different films and the length of exposure to see what the relative effects of those factors _might_ be on any reciprocity "expansion" of contrast, and against which to compare any empirical tests I do in the future. (Yeah, I read too much Henry James. ;-)

    In other words, I can graph the difference between the Bond data filter factor of 0.101 for HP5+ and the Ilford chart factor of 0.51, and do it at different exposure lengths while monitoring generalized contrast effects. I don't expect it to be deadly accurate, just generally indicative, providing a visualization of the concepts rather than hard data to shoot from and expect to print on grade 2.0001 paper. The spreadsheet does agree with the Bond tests in indicating less expansion of contrast with newer films compared to the older emulsions.

    I really could have used the information we've been swapping here when I was doing some quick pinhole work last spring with a chart based on the generic film charts and got some really overexposed XP2 negatives. Luckily, I can reshoot.

    I've heard the same things that Kirk Keyes mentions about Acros having very low reciprocity failure. I have some on hand to try. See the chart on the Ryuji Suzuki web page I mentioned earlier to see how it compares to several other films.

    Lee

  7. #87

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    Lee, please do check your Acros - I've also seen mention of it's excellent long exposure properites. I was using an enlarger and step wedge, and I wasn't able to do any longer tests since I didn't have time to enclose the enlarger head to cut down more on the stray light... but I would like to see results from someone that obviously understands the subject.

    Kirk

  8. #88
    Lee L's Avatar
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    Acros reciprocity number

    Quote Originally Posted by Kirk Keyes
    Lee, please do check your Acros - I've also seen mention of it's excellent long exposure properites. I was using an enlarger and step wedge, and I wasn't able to do any longer tests since I didn't have time to enclose the enlarger head to cut down more on the stray light... but I would like to see results from someone that obviously understands the subject.

    Kirk
    Kirk,

    I'm just beginning to understand. There are lots of people who know a lot more than I.

    It may be a while before I run the Acros, (I've got a number of films I want to test in several developers, having been out of the darkroom a while) but I've calculated the Acros film factor for the Gainer formula from the mfgrs data sheet and the chart at Ryuji Suzuki's site, and it's in the neighborhood of 0.019 if that will help you. The data sheet is at: http://tinyurl.com/689a8 (sorry, the original was very long and had a sessionid) It basically says to open up 1/2 stop for any exposure between 120 and 1000 seconds.

    I'll post when I get something myself. I'll definitely load the Acros when I need long times. I'd try it for astrophotos if it didn't drop off the map below 650nm, which makes it pointless for one of the common wavelengths in nebulae emitted by hydrogen. It might work fine for some wide constellation shots without nebulae.

    Lee

  9. #89
    gainer's Avatar
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    I'll post when I get something myself. I'll definitely load the Acros when I need long times. I'd try it for astrophotos if it didn't drop off the map below 650nm, which makes it pointless for one of the common wavelengths in nebulae emitted by hydrogen. It might work fine for some wide constellation shots without nebulae.

    Lee

    I think you meant to say above 650 nm. Below 650 is most of the visible range. Hydrogen IIRC is in the red with lines above 650. I'm pretty sure Acros records visible light.
    Gadget Gainer

  10. #90
    Lee L's Avatar
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    Quote Originally Posted by gainer
    I think you meant to say above 650 nm. Below 650 is most of the visible range. Hydrogen IIRC is in the red with lines above 650. I'm pretty sure Acros records visible light.
    Sorry if I was unclear. Perhaps I should have phrased it "before reaching 650nm". I was mentally going "up" in wavelength as opposed to frequency, a so the drop off in sensitivity in that case would be "before" or "below" 650nm. In any case, as you've noted, Acros is meant for the visible spectrum (at least most of it), and sensitivy doesn't extend to 650nm or "above". The IR folks would have a ball with it if it "saw" only above 650nm. And yes, you recall correctly, the H-alpha line is near 656nm.

    Lee
    Last edited by Lee L; 01-14-2005 at 02:58 PM. Click to view previous post history.

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