Tonal effects from reciprocity

Discussion in 'B&W: Film, Paper, Chemistry' started by Jan Brittenson, Sep 22, 2003.

  1. Jan Brittenson

    Jan Brittenson Member

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    I was wondering if anyone has investigated the results of reciprocity on the tonal curve. Say I read a Zone V exposure (middle of the range) at 1 min. Reciprocity corrected, it might be 4 min. (Just making up numbers here, they're not important.) Now, Zone II is going to be 3 stops further up along the reciprocity curve (the 8 min point), meaning there likely won't be any density there on the film. Zone VIII on the other hand, is way down in terms of reciprocity (at the 1/8 point, likely not even affected), and will likely end up blocked. So I surmise there's both an exposure shift and a folding of the reciprocity curve into the tonal curve. Any thoughts on this? Am I correct here? Anyone using it to good effect?
     
  2. lee

    lee Member

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    I would think that this is one reason that one sometimes, with certain films, needs to adjust the time in the developer. I would not be reading the zone V anyway. I would be working with zone III or zone IV. Then, depending on the reciprocity curve for that film, I would develop accordingly. If the zone V tone was the important one I might do as you suggest and let everything else fall where it may.

    lee\c
     
  3. Lex Jenkins

    Lex Jenkins Member

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    My observations are based on experience with a limited selection of materials: Tri-X in Diafine and stand processed in Rodinal; FP4+ in Diafine.

    Under full moonlight I see fairly normal tonal characteristics (assuming I get the exposure right, which isn't easy). In fact, some of my Tri-X exposures when souped in Diafine could almost pass for daylight photos.

    Shadow detail can be quite good, midtones can be very normal and highlights tend to be blocked up only where absolutely unavoidable, such as moonlight reflected on the water.

    No idea how these films might behave with more typical developers and more "normal" development - haven't tried.

    And I wouldn't be at all surprised if objective testing totally refuted my casual observations. I judge my negatives solely by how they print and nothing else. If they "look" like nighttime photos while retaining the tonal characteristics, shadow and highlight detail I want, I'm happy.
     
  4. dr bob

    dr bob Member

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    Jan & Lee:

    You have a point, and you exhibit a depth of technical thought. I have found that low intensity lighting creates a very contracted zone negative. This is due to the very effect you mentioned. In low illumination, one usually bases his exposure on the shadow, or rather the lowest needed zone (as the shadows are aped to be essentially black). This should cause the higher zones to receive correspondingly more exposure than they would receive under “normal” lighting. I do (did) much night and low light level photography. I even got the title of “unavailable light photographer” at the local art group. I do a lot of “-N” development, but I think my best efforts were produced with split D23 developer. It is possible to expand the gray scale three zones this way if one is careful and applies experimentation.
     
  5. lee

    lee Member

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    N- development with D23 split is quite useful. I spent about a year with Microdol-x which is close to D-25 and used Kodalk as the second bath and some of the reciprocity issues were solved for me this way. Dr Bob is right about the 3 zone expansions. It can be a very useful tool.

    lee\c
     
  6. Donald Miller

    Donald Miller Member

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    If one had a low illumination scene in which the brightness range were 5 zones (for instance) and the lowest level were placed at a zone III at a time of exposure of one minute (using the example given). Allowing for reciprocity the adjusted time would be four minutes. The next zone above (IV) would have metered thirty seconds and received four minutes (approximately 200% overexposure after allowance for reciprocity). The placement for Zone V would have metered fifteen seconds and received four minutes (approximately 400% over exposure after allowance for reciprocity). The placement for Zone VI would have metered 7.5 seconds and received 4 minutes (approximately 1600% overexposure). The placement for Zone VII would have metered 3.75 seconds and received 4 minutes (approximately 3200% overexposure). The fifth zone would metered 1.875 seconds and recieved 4 minutes (an approximate 6857% overexposure).

    All calculations above are on the basis of a Zone V rendering for all zones (obviously Zones placement VI through VIII would have received more exposure to rendered their desired negative density. But the percentage increase would have been a 100% increase above the preceding zone luminance as opposed to the precentages given.

    The other way of dealing with reciprocity and the matter of contracted development is to support the low densities through preflashing the film at a uniform non image bearing Zone III luminance. The luminance range is still being contracted. However they are contracted in a different way. In the case of N minus development the high densities are contracted in order to arrive at desired low value exposure. In the case of preflasing of film the low values are supported prior to exposure to the point that reciprocity is not the factor that it is in the example given.

    In other words through preflashing we are working on the support of low value exposure so that we may not need the extreme contraction that we would be required to use if only N minus development were used.
     
  7. clay

    clay Subscriber

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    Low light negatives will display increased contrast precisely for the reasons you said. So when you underdevelop the neg to compensate, you lose even more film speed, and slip further into reciprocity territory, and gain even more contrast. It's not just a single variable problem (i.e. exposure time), because the variable of development time is a factor as well.

    If you want an easy answer to your question, just leverage the amazing amount of work that Phil Davis did last year and buy a palm pilot and put the BTZS expodev program on it. His film development curve data handles many different films and developer combinations. What is interesting is that not all films behave the same in different developers. The real champ from the huge amount of research he did was HP-5/Ilford DDX. It loses less speed (and consequently gains less contrast) than just about any other conventional film combo. But I can attest from personal use of this program that it works, and works extremely well. Because I shoot 12x20 film, I don't bracket, and his little program lets me do it with confidence. Never had a negative that wasn't pretty close to just right.

    Clay
     
  8. Black Dog

    Black Dog Member

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    Hmm...haven't noticed anything like this before, despite some LONG exposures (up to 16 hrs on Polaroid 55). Normally, I use PMK or a 2 bath dev which seem to be really good at keeping a lid on highlight density.
     
  9. Lex Jenkins

    Lex Jenkins Member

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    One reason I use Diafine for developing my nighttime stuff is to reduce the contrast problem. Works too. So does stand development in dilute Rodinal. Got examples of each in my goat skull photos uploaded here.

    Dunno the why's and wherefors of each, tho' - don't have a densitometer.

    Don't get me wrong - I'm not raising a glass to ignorance. I'm just happy to acknowledge what works for me.
     
  10. Black Dog

    Black Dog Member

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    Yeah, me too-if it prints ok (so far so good) it is ok.