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Thread: Reciprocity - how far?

  1. #1

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    Reciprocity - how far?

    Does Reciprocity for long exposures keep going on - forever?

    Or does it go "so far" and then just kind of stop. Not going past a certain point and after that it is basically done?

    Asking after looking at some 8x10 negatives and prints that were done with 2-8 hour exposures. Very low light in a soon to be demolished jail. One 40 watt light was the source and it had been blocked heavily with wet toilet paper, apparently so the resident could sleep. Very little light in the place as the hallways were single fluorescents widely spaced.

    The photographer metered the scene and the reciprocity chart didn't go long enough so he guessed and left to use smaller formats in other cellblocks while these exposures cooked. Per him, every one turned out well, printable and no problems along the way.

    The negative looks very good and the prints even better.

    So, exposures of many minutes to hours - some "slop factor" in the mix. Not like 1/125 second at all.

    Any information about very long exposures and reciprocity you can direct me to?
    "My forumla for successful printing remains ordinary chemicals, an ordinary enlarger, music, a bottle of scotch - and stubbornness." W. Eugene Smith

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    Re: Reciprocity - how far?

    Which film?

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    Re: Reciprocity - how far?

    Reciprocity factors do seem to continue. When Ray McSavaney made his beautiful images in the abandoned Royal Tire factory with no light other than the small amount sneaking through the small high windows he used Tri-X and made exposures several hours long. Te films were developed in very dilute HC110 in trays.

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    Re: Reciprocity - how far?

    In this document...

    https://www.ilfordphoto.com/wp/wp-co...mpensation.pdf

    ...Harman/Ilford provides an exponential formula for calculating reciprocity, with a table listing the exponent to be used for each of their films. There's no indication of how far out you have to go in exposure before the formula starts to fail.

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    Re: Reciprocity - how far?

    The film used was Ilford FP4+.

    While the 8x10 and 5x7 cameras were exposing film he was shooting in cell areas with 35mm TechPan. Much faster lenses and the contrast of the film was a nice boost to get prints that looked very good.
    "My forumla for successful printing remains ordinary chemicals, an ordinary enlarger, music, a bottle of scotch - and stubbornness." W. Eugene Smith

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    Re: Reciprocity - how far?

    I suspect that the exponential reciprocity rule would continue up to the point where no amount of exposure would create an image -- i.e. not enough photons to affect a silver molecule.

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    Resident Heretic Bruce Watson's Avatar
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    Re: Reciprocity - how far?

    Quote Originally Posted by Willie View Post
    Does Reciprocity for long exposures keep going on - forever?

    Or does it go "so far" and then just kind of stop. Not going past a certain point and after that it is basically done?
    What low intensity reciprocity failure (LIRF) is about is the film not capturing enough photons to create a latent image. No latent image == nothing for the developer to develop.

    The process of creating a latent image is not inherently stable. That is, the creating of metallic silver through the action of absorbing a photon, kicking out an photoelectron, and reducing a speck of silver ion into metallic silver is not permanent. Eventually the metallic silver will convert back into silver ion. And that tiny bit of your latent image will be lost. The question is how long does it take to revert?

    And the answer to your question is clear -- there is a point where continued exposure is pointless. That's the point where you are creating new silver at the rate that you are loosing the silver you previously created. When you aren't creating more silver than you're losing, that is creating more latent image, you are done. Further exposure will create no new latent image.

    This is why the reciprocity failure curves from the film manufacturer are exponential curves, and not linear. If it were linear, it would go on forever. But since it's an exponential curve, it's easy to see that it becomes asymptotic to the efficiency of the film at capturing photons and maintaining the resulting metallic silver. Every film will be different, but they will all have a point at which the capture of photons and the creating of metallic silver balances with the reversion of metallic silver back to silver ion state. When you reach this point, further exposure is futile. As you say, you are done.

    Now you know why older cubic grained films have considerably worse reciprocity failure characteristics compared to more modern tabular grained films -- the tabular grained films are more efficient, and therefore have better reciprocity failure characteristics. Said another way, they can pull more shadow detail from a given long exposure.

    This shows up all the time -- not just in poorly lit "about to be scraped" jails. It happens in brightly lit scenes that have deep shadows too. I've had this problem (pulling shadow detail from areas that were really dark) in mountains streams in full sunlight, with a 1/15 second exposure. The rock I was interested in was somewhat backlit, and the water in front of it was, well, water, and reflecting what little light it got every which way it could but back to the base of the rock. I spend days and not a few trips back trying to get everything I wanted to show on the film, but Tri-X wasn't having it. It wasn't until I switched to TMY a couple of years later that I found I could effortlessly capture that shadow detail -- got a perfect exposure of that damned rock on my first try (and after all that work I never printed it; I guess I was just over that particular image by then, sigh...). And it was only then that it dawned on me how reciprocity failure actually worked, and what it meant to my photography. Needless to say I dumped the rest of that box of Tri-X in the darkroom trash can and never used anything else buy TMY after. But that's because that's the tool I needed to do the work I was trying to do. Tri-X is a fine film. But TMY has better reciprocity failure characteristics for what I was trying to accomplish.

    Bruce Watson

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    Re: Reciprocity - how far?

    I'm sure there must be some upper limit where there's just not enough light to form an image, but I once did a 3-hour pinhole exposure that printed just fine.

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    Mark Sawyer's Avatar
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    Re: Reciprocity - how far?

    Quote Originally Posted by Bruce Watson View Post
    The process of creating a latent image is not inherently stable. That is, the creating of metallic silver through the action of absorbing a photon, kicking out an photoelectron, and reducing a speck of silver ion into metallic silver is not permanent. Eventually the metallic silver will convert back into silver ion. And that tiny bit of your latent image will be lost. The question is how long does it take to revert?
    I'm not sure about the reversion of the silver back to an ionized state. 75-year-old unprocessed film from WWII can still hold a decent image, (https://petapixel.com/2015/01/16/31-...red-processed/) and 100-plus-year-old film from the Shackleton Expedition was successfully processed.

    My understanding, though I could be wrong, is that 1000 photons dribbling in one at a time don't have the same effective force of 1000 photons hitting all at once.
    "I love my Verito lens, but I always have to sharpen everything in Photoshop..."

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    Re: Reciprocity - how far?

    Quote Originally Posted by Mark Sawyer View Post
    I'm not sure about the reversion of the silver back to an ionized state. 75-year-old unprocessed film from WWII can still hold a decent image, (https://petapixel.com/2015/01/16/31-...red-processed/) and 100-plus-year-old film from the Shackleton Expedition was successfully processed.
    The difference is that those films were exposed at regular amounts of light -- guzzillion times brighter than when extreme reciprocity kicks in.

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