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Thread: Reciprocity Law Failure

  1. #1

    Reciprocity Law Failure

    Could somebody explain this to me? I understand what it is, increasing exposure times as film does not react to light in the way it was intended to at longer exposures. I looked at Ilford's chart for calculating this, but I dont understand it. Is it just a case of doubling the stop by what it would normally be? I am planning on shooting 5x4 film at quite a slow speed, so I might have to do longer exposures to what I normally do, depending on the circumstance.

    Thank You

  2. #2

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    Re: Reciprocity Law Failure

    https://www.google.com/search?q=reci...&client=safari
    Read first search result under chemical photography. It's explained very clearly.

  3. #3

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    Re: Reciprocity Law Failure

    The exposure required to reach the same result on film is a bowl or bow-shaped curve, where the bottom of the bowl is somewhat flat in the time range the film is designed for... For example between 1/1000 second to 1 second.

    As the curve bows up, the necessary exposure goes up logarithmically, so it's not necessarily doubling. Each film has its own reciprocity characteristics, so when you find a chart for your film, it may not apply to other film. (So you may need to draw up a different chart for different films)...

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    Re: Reciprocity Law Failure

    You might find it helpful to search the forum for earlier discussions of this topic.

    Here's a graph which shows that up-swept curve, provided by another forum member.

  5. #5

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    Re: Reciprocity Law Failure

    Lately I've been experimenting with dimly lit subjects and generous bellows extension: the exposures get very long, like a minute or more.

    I have found this iPhone app very helpful, because it takes everything into consideration: Reciprocity Timer. It paid for itself with the first few sheets of film it saved.

  6. #6
    rich815's Avatar
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    Reciprocity Law Failure

    As others aptly point out you do extend exposure time above and beyond what you would following a standard linear exposure chart however if you consider using Fuji Acros film no adjustment is required for exposures up to 120 seconds, and only +1/2 stop additional for exposures between 120 and 1000 seconds. It's a wonderful film.

  7. #7

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    Re: Reciprocity Law Failure

    When I started using Ilford film I noticed that the same exact curve for reciprocity was published for several of their films, so I queried them about that at the time. They responded by saying that they are a small company (compared to Kodak?) and could only provide a rough approximation rather than performing lengthy tests on each film, and a serious worker should perform personal tests on the film of choice. I found in AA's "Negative" book that he referenced Kodak's recommendations, which indicated the need to reduce dev times as the length of exposure grew in order to offset the resulting gain in contrast; AA explained the cause of the contrast increase. I've yet to see any author mention if choice of developer/dilution is a factor. Considering the variables involved, I've concluded that the serious worker who frequently works under reciprocity conditions is better off to go with Acros and keep the testing $ in his pocket to buy more film.

  8. #8
    photobymike's Avatar
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    Re: Reciprocity Law Failure

    Quote Originally Posted by Ken Lee View Post
    Lately I've been experimenting with dimly lit subjects and generous bellows extension: the exposures get very long, like a minute or more.

    I have found this iPhone app very helpful, because it takes everything into consideration: Reciprocity Timer. It paid for itself with the first few sheets of film it saved.
    Holy Crap ...my iphone is even more useful thanks Ken!!

  9. #9
    Dave Karp
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    Re: Reciprocity Law Failure

    I found that for the Ilford films included, the reciprocity chart in Steve Simmons's view camera book work for me (and different than Ilford's recommendations).

  10. #10

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    Re: Reciprocity Law Failure

    Quote Originally Posted by Ken Lee View Post
    You might find it helpful to search the forum for earlier discussions of this topic.

    Here's a graph which shows that up-swept curve, provided by another forum member.
    Thanks Ken,

    A piece of paper with a curve. That's what I need, since I don't have an intelligent phone...

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