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Thread: Reciprocity - how far?

  1. #11
    Steve Sherman'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?

    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?
    I do a lot of long exposure photography, in the early days using McSavveny's unusual sequence of developing with HC 110. Today, the HC 110 is a poor developer for that type of extended time in chemistry. There is a host of reasons why Pyro developers are perfectly suited for long times in solution. After trying several notable photogs reciprocity suggestions I have found the Ilford factor system to be quite accurate, possibly a touch too much exposure when you get into more than 60 minute exposure times.


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  2. #12
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    Re: Reciprocity - how far?

    Quote Originally Posted by xkaes View Post
    The difference is that those films were exposed at regular amounts of light -- guzzillion times brighter than when extreme reciprocity kicks in.
    So properly-exposed long exposures won't last if left unprocessed, but properly-exposed short exposures will?
    "I love my Verito lens, but I always have to sharpen everything in Photoshop..."

  3. #13
    Resident Heretic Bruce Watson's Avatar
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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.
    Kodak put in a lot of work over the decades to stabilize the latent image and increase the time it would stay usable between exposure and processing. I'm fairly sure that TMY is better at this than SUPER-XX was for example. Much probably has to do with storage. Cooking exposed film in the truck of a car in the desert sun probably would give worse results than storing it in a freezer for example.

    That said, the image itself is made up of trillions+ silver molecules. If you loose a few, will you notice? Which brings us to how does one define "a few"? I suspect that the image processing people can tell us, and we know that film typically records far more detail than needed to form a convincing image. But how much has to remain to be acceptable? IDK.

    Quote Originally Posted by Mark Sawyer View Post
    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've heard this too but have no idea how true it is. I sort of doubt it because of the stochastic nature of the process. But I'm not the guy with an answer to that one.

    What I'm relatively sure of is that not every photon that passes through the emulsion is "caught". I'm not sure what the percentage is, but I suspect it's actually fairly low. That is, most of them go right on through the film unaffected. I would think most of these are "mopped up" by the anti-halation layer. That is largely the purpose of the anti-halation layer isn't it?

    The bottom line remains that you have to expose the emulsion with sufficient photons to form a latent image or the developer has nothing to develop. I don't know what "sufficient" means in this case beyond that at least some are required to interact with the emulsion and kick off the chemical processes. Grant Haist would probably know, and he might talk about it in his masterwork Modern Photographic Processing. IDK, I haven't see the books in probably decades now.

    Ah, OK, how about this? Without sufficient photons we end up with a clear spot on the film. With enough photons we end up with some image detail, fewer photons giving us shadow detail, more photons giving us highlight detail. Too many photons and we overwhelm the film and end with with a black spot (a specular highlight?) on the film. The art of exposure is in picking the right number of photons.

    As to reciprocity failure, this seldom happens to an entire sheet of film at the same time. Far more typically is that parts of the film experience reciprocity failure while the rest do not. This is what makes night shots so difficult -- the streetlight on the bridge over the river in the park gets seriously over exposed in the quest for longer exposures to give some shadow detail to the bridge supports that extend down into the river. Thus the linking of reciprocity failure to increased contrast. Sort of a misnomer really, it's more about overexposure than increased contrast. But increased contrast is what people see in a print.

    Bruce Watson

  4. #14

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    Re: Reciprocity - how far?

    This is Wikipedia's explanation of reciprocity failure:

    At very low light levels, film is less responsive. Light can be considered to be a stream of discrete photons, and a light-sensitive emulsion is composed of discrete light-sensitive grains, usually silver halide crystals. Each grain must absorb a certain number of photons in order for the light-driven reaction to occur and the latent image to form. In particular, if the surface of the silver halide crystal has a cluster of approximately four or more reduced silver atoms, resulting from absorption of a sufficient number of photons (usually a few dozen photons are required), it is rendered developable. At low light levels, i.e. few photons per unit time, photons impinge upon each grain relatively infrequently; if the four photons required arrive over a long enough interval, the partial change due to the first one or two is not stable enough to survive before enough photons arrive to make a permanent latent image center.

    https://en.wikipedia.org/wiki/Reciprocity_(photography)

    btw, "Reciprocity also breaks down at extremely high levels of illumination with very short exposures. This is a concern for scientific and technical photography, but rarely to general photographers, as exposures significantly shorter than a millisecond are only required for subjects such as explosions and in particle physics, or when taking high-speed motion pictures with very high shutter speeds (1/10,000 sec or faster)."

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    Re: Reciprocity - how far?

    There is no way to know which unknown "expert" did what research before posting on Wikipedia. I don't believe any posting there unless by a known expert whom I trust to have done proper research.

  6. #16
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    Re: Reciprocity - how far?

    But it is what I was told back in the 80s before Wiki-anythings (except for the wikiups the hippy back-to-the-landers built in the 70s). I think it was at a Friends of Photography workshop lecture by John Sexton.

    Generally the limit to long exposures tends to be dawn. I have failed to wake up before first light once or twice.

    Also interesting that an exposure made of multiple (electronic) flashes, is less than the sum of the flashes.
    "Landscapes exist in the material world yet soar in the realms of the spirit..." Tsung Ping, 5th Century China

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    Re: Reciprocity - how far?

    Quote Originally Posted by Jim Noel View Post
    There is no way to know which unknown "expert" did what research before posting on Wikipedia. I don't believe any posting there unless by a known expert whom I trust to have done proper research.
    wikipedia's right, that's more or less how it works.

  8. #18

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    Re: Reciprocity - how far?

    "There is no way to know which unknown "expert" did what research before posting on Wikipedia. I don't believe any posting there unless by a known expert whom I trust to have done proper research."

    It is fine to be skeptical of course. But why stop there? the wiki page provides citations and therefore the source of the information. They are journal papers or technical books.

    For example, one interesting reference can be found here:
    https://physicstoday.scitation.org/d....1063/1.881181

    Another interesting source is in part II-section VI of the reference:
    https://ia802701.us.archive.org/33/i...01meesgoog.pdf

    and also
    https://articles.adsabs.harvard.edu/...00089.000.html

    You can see that this has been documented for over 100 years. There may be more 'direct' sources that address the underlying physics of the reciprocity failure but I haven't seen a 'final paper/chapter book' on this front. If someone knows of one that is the 'ultimate closure' on this topic, I think we'd be interested. But I'm afraid it's probably dispersed throughout many sources.

  9. #19

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    Re: Reciprocity - how far?

    Quote Originally Posted by Vaughn View Post
    ... Also interesting that an exposure made of multiple (electronic) flashes, is less than the sum of the flashes.
    The intermittency effect, which also affects stacked exposures made with natural light and a regular shutter. Eight 1/8-second exposures won't quite equal a single one-second exposure. The effect is rather small and not important most of the time. Still, when I'm stacking exposures, I'll often add a bit just to err on the side of a bit of overexposure.

    An interesting thing about low-intensity reciprocity failure (LIRF if you will) is that the amount of failure is proportional to the brightnesses in the subject. Lower values suffer more than the brighter ones effectively stretching out the exposure range. I find pushing film into reciprocity failure with a neutral-density filter to get exposures of one to several minutes is a good way to expand contrast in a way that looks much different than extending development or printing contrastier (providing, of course, that one gives adequate exposure to get the shadow detail one desires).

    Also, keep in mind that an hour or two extra exposure in a multi-hour exposure ends up being a fairly small overexposure. If you need eight hours to get the right exposure (after figuring compensation for reciprocity failure, of course) then another two hours is only a 25% overexposure; not even a half a stop.

    Doremus

  10. #20

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    Re: Reciprocity - how far?

    I may add that one thing that puzzles me is that the degree of reciprocity dependence of the film brand/type can be quite large. Some films can go for like 2 min without exhibiting reciprocity failure whereas some exhibit it after 1 or 2 seconds (one of the reasons I personally like Provia over Velvia, for example, in low light conditions). Therefore it would seem that the characteristics of the film have a huge range of influence (like 2 orders of magnitude in time).

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