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  1. #11
    Helen B's Avatar
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    I also think that there is no alternative but to do tests. Presumably each film has a point at which the intensity of light falling on it is too low to form a stable latent image - so there is a danger that in designing for an extremely long exposure time you could end up with no image. Extreme reciprocity failure resulting in no viable image centres except in the brightest parts of the scene. This can only be determined by testing, surely.

    Best,
    Helen

  2. #12
    Bob F.'s Avatar
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    I would also suggest that you trial it. Meter shadow & highlights in the station during the day and set up in your bedroom/garage/etc with the same lighting levels and burn a few rolls of film. That will let you hone in your development times too as the kind of reciprocity error you are hitting is going to do interesting things to the film curve...

    Good luck getting a permit if you are going to be where people need to walk past - I'd concentrate on getting that first.

    Cheers, Bob.

  3. #13
    Helen B's Avatar
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    I don't know why I didn't think of this before, but the obvious answer is to use a development technique that dramatically reduces the sensitivity of the film, rather than decreasing the intensity of the light falling on the film. This gets round the extreme reciprocity failure problem, but it could lead to other problems.

    So folks, what suggestions? Open the aperture and see if you get an image without development (ie raise the intensity of light on the film)? Probably not enough. Post fixation physical development?

    Best,
    Helen

  4. #14
    Karl K's Avatar
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    Eight Hour Exposure continued

    Sorry, guys, I thought I made it clear. The basic (no filter) light meter reading is four seconds @f/22, with 200 ASA. I am using the Ilford SFX filter, which has a filter factor of 16. I want to make the exposure 8 hours. Why? Because that will eliminate all traces of movement. I can do the math for a filter factor of 16. I can't figure out the reciprocity and the filter factor of 16 together. I also have available the proper neutral density filter, should that become necessary to use. That would make the calculation even more complicated. Basically, I'm worried about receiprocity for such a long exposure. Ilford says that they don't recommend that SFX be exposed for eight hours, but I want to give it a try anyway.

  5. #15

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    You don't need 8 hours to eliminate the movement. Having done extensive shooting in cathedrals with ULF cameras I speak with some experience. Needing shadow detail in the ceilings necessitated exposures that averaged 4 hours. One five hour shot had the janitor buffing the center aisle for ~2 hours and he never showed at all.

    Also, there is a difference between a 3 and a 5 hour exposure for shadow detail. After 5 hours the reciprosity never seems to catch up with itself and additional time was of limited value.

    I was using Bergger BPF 200 at the time and have no experience with SFX. Good luck and don't be afraid to test out the responses before an important shoot. Oh, yeah, and get a really sturdy tripod.

  6. #16

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    In my eyes, not all the cameras and films are intended to be used with such exposure times - I am thinking about light scattering inside the camera. In eight hours it can be a real foggy trouble using a 35mm camera. With LF and ULF the things are way much simpler thanks just to the bigger volume of bellows, and effective absorption of scattered light by bellows' ridges. But Leica R, 135 film with its less-than-perfect antihalation layer, and extra-wideangle lens... to me, it just doesn't seem to be a proper tool set for the job. Maybe I am wrong, of course.

    Zhenya

  7. #17

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    Quote Originally Posted by Karl K
    Sorry, guys, I thought I made it clear. The basic (no filter) light meter reading is four seconds @f/22, with 200 ASA. I am using the Ilford SFX filter, which has a filter factor of 16. I want to make the exposure 8 hours. Why? Because that will eliminate all traces of movement. I can do the math for a filter factor of 16. I can't figure out the reciprocity and the filter factor of 16 together. I also have available the proper neutral density filter, should that become necessary to use. That would make the calculation even more complicated. Basically, I'm worried about receiprocity for such a long exposure. Ilford says that they don't recommend that SFX be exposed for eight hours, but I want to give it a try anyway.
    Reciprocity failure is usually applied after the metered exposure is known. You're going backwards so if you want an eight hour exposure you adjust for reciprocity from this eight hour value to your metered exposure ignoring all filter factors. You then divide this number by your 4sec base exposure to give you the total filter factor needed. You can then divide this by 16 to adjust for your SFX filter which will give you the ND filter factor required.

    The problem is that I doubt that many (if any) have any notion of what reciprocity factor to apply in this case. Ilford don't publish it on their SFX datasheet and I personally haven't had any exposures with SFX longer than about 10 seconds. You will need to be aware that the reciprocity failure adjustment (usually log(-1/x) order) may be infinite for the times you're talking about in which case it cannot be done. Another possibility at these time scales is that an infinitely small change in x (metered exposure) correlates to a significantly large change in y (adjusted exposure) which means a very small filter factor adjustment can be the difference between a completely blank negative and a completely overexposed negative. Because you're dealing with severe reciprocity you may find that somewhere in the middle of these you end up with a lith negative (ie areas which have had light above a certain threshold are completely exposed and anything below are completely unexposed).

    Put simply you're going to have to do tests unless you're lucky enough to find someone who's done exactly the same thing with exactly the same film.

    Roger.

  8. #18

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    Quote Originally Posted by David
    Oh, yeah, and get a really sturdy tripod.
    I'm pretty new to this so I was wondering...I've spent a lot of time in Grand Central. With all the people and the trains the floor does vibrate quite a bit. Wouldn't that affect the sharpness of the image or does the long exposure and/or tripod negate that?

    ..john

  9. #19
    Helen B's Avatar
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    Quote Originally Posted by David
    ...

    Also, there is a difference between a 3 and a 5 hour exposure for shadow detail. After 5 hours the reciprosity never seems to catch up with itself and additional time was of limited value.
    ...
    That's exactly the point I was trying to make. For every film there is a light intensity that will never create a latent image no matter how long you expose the film for. What is happening, highly simplified, is that silver atoms are being created in such low numbers at such a low rate that they do not stabilise - they die a lonely death before they find a mate to help them survive.

    Best,
    Helen

  10. #20
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    A quick consultation of the Ilford's reciprocity tables, 64 seconds doesn't even show up. In fact, the curve may approach 64 seconds asymptotically close, but might never actually get there. This has already been stated very emphatically by others here, but it is an important fact. The curve flattens out so much at the extremes that any increase in the exposure time will not affect density at all. Change your aperture to something smaller.

    Drew
    "But what is strength without a double share of wisdom." --John Milton

    "Our greatest fear should not be of failure, but of succeeding at something that doesn't really matter." --Unknown missionary

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