B&W film reciprocity calculations

Discussion in 'B&W: Film, Paper, Chemistry' started by spoolman, May 14, 2008.

  1. spoolman

    spoolman Subscriber

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    Hello All:I'm finding the exposure calculations for reciprocity failure for most films to be not very accurate.Is there a formula or calculation that can be used to more accurately pinpoint the adjusted exposure time for most black and white roll and sheet films.

    Thanks,

    Doug:smile:
     
  2. Ray Heath

    Ray Heath Member

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    g'day spool

    why do you think the calculations are not very accurate?

    what result do you get?

    what result do you want?

    Ray
     
  3. gainer

    gainer Subscriber

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    They're like women: all the same but a heck of a lot different, thanks be.

    The reciprocity behavior of all the films for which I have seen data fall on parallel straight lines on log-log graph paper. That is the way they are both all the same, yet all different. Look in www.unblinkingeye.com for an article called "LIRF is Lurking at your F-Stop" by some guy named Gainer. LIRF stands for "low intensity reciprocity failure."

    The fact that all their lines are parallel means that if you know accurately one point on the line you can draw the chart for all the others. The slope of the line is 1.62 on log-log graph paper.
     
  4. Ian Grant

    Ian Grant Subscriber

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    Your best bet is download the film manufacturers data sheets. They list the reciprocity adjustments needed. There isn't a common calculation that holds good for all films,Tmax films behave quite differently to conventional films.

    A lot can also depend on the accuracy of you light meter in low light levels.

    Ian
     
  5. Lee L

    Lee L Member

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  6. gainer

    gainer Subscriber

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    The problem is usually in trying to write an equation that describes the entire exposure instead of the part that differs from what the meter reads. If the equation only describes the additional exposure needed to correct for reciprocity "failure" then even the TMax and Ilford T-grain films are parallel on log-log graph paper.
     
  7. Peter Williams

    Peter Williams Member

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    You can always just shoot Fuji Acros and stop worrying about reciprocity forever.
     
  8. RobC

    RobC Member

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    if you need to refer to reciprocity charts then it means that you are working in low light or night time. When you are doing that, the way you think about your image changes. For example, a deep deep shadow placed on zone II or III will probably be way brighter in the result than it was in reality. So, if you want to retain the low light or night time appearance of the scene in your final result, it is better to expose for the highlights and let the shadows fall where they fall. Also only adjust for reciprocity based on your highlight values.

    So meter something you want on a zone VII or VIII and work out the exposure to place it on that zone and then if that exposure is into reciprocity, adjust for that time.

    I have found that this works really well using HP5+ published reciprocity figures.

    If you meter a deep shadow and try and place it on zone II or III you will invariably be making it too light and by doing so you are giving it too much exposure and the exposure required to do that is a long way into reciprocity and when you correct for that, you push the hightlights off the end of the curve. So meter and expose for highlights when doing low light/night time photography and use manufacturers recommended figures for reciprocity correction.

    To recap: Your judgement about zones or placement of values in low light conditions is screwed by your subconcious perception of how things should be on film based on daylight conditions. Metering and exposing for highlights remedies this(mostly).
     
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  9. Nicholas Lindan

    Nicholas Lindan Advertiser Advertiser

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    Reciprocity failure is proportional to light intensity. There is more failure in the shadows than in the highlights and so the contrast will increase. You need to meter and reciprocity correct the shadows. Developing for the highlights then has to compensate for both the scene luminance range and the lessened reciprocity failure in the highlights.

    A film that is well characterized for reciprocity failure will give a list of exposure time increases and corresponding reductions in development time. This will be based on the standard 7 stop range and different subjects will require different treatment.

    The three factors for success are experimentation, experience and luck. Pretty much like the rest of life.
     
  10. keithwms

    keithwms Member

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    I discovered some time ago that the astronomers used to trade very reliable and handy reciprocity tables.

    Anyway look here:

    http://mkaz.com/photo/tools/reciprocity.html

    (which has several links at the bottom. N.b. the Fuji 64T in this chart is not the current 64T which has somewhat better recip. characteristics)

    At some point I just put a bunch of these on my phone. I decided it was easier than working out a way to run a calculation on the thing.

    Incidentally, I use reciprocity charts for night stuff but more often for macro. Take 64T to 5:1 indoors and you quickly see why (even though it has quite remarkable reciprocity).
     
  11. Rich Ullsmith

    Rich Ullsmith Subscriber

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    The thing I have found about dealing with reciprocity failure, with any film, is that even a 1/4 stop difference in metering will compound in the calculations. It can result in several seconds to several minutes difference in the final exposure. It almost doesn't matter how accurate the data sheets are in regard to reciprocity failure, because they get their metering data in clinical conditions, and I get mine outside with clouds passing by. Experimentation, experience and luck, indeed.
     
  12. gainer

    gainer Subscriber

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    Theories about how reciprocity fails need corroboration by experiment, just as Einstein did. It might be worthwhile to read Howard Bond's article in Photo Techniques of a few years ago, in which he tested issues of change of gradation due to long exposures. I used his data as the base for my article in the following issue, which is also in Ed Buffaloe's Web site. He found that there was no significant effect on gradation tha could not be corrected by development time or a change of paper grade. If there is such an effect, it cannot be corrected by altering the exposure time. That is the nature of the effect to begin with. If using a long exposure time causes a change in effective exposure which in turn causes a change in gradation, how will an additional exposure reverse the change in gradation? The best bet is to base your exposure on the darkest detailed shadow, as usual, and correct gradations if need be by adjusting development time, or by adjusting printing contrast. Bond's favorite method is the unsharp masking technique, which he seems to use at every opportunity, and to good advantage at that.
     
  13. RobC

    RobC Member

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    I urge people to try exposing for the highlights when in low light or night time photography. Don't take my word for it, just try it for yourself. I know this is at odds with the old adage "expose for shadows and develop for highlights" but getting those mid to highlight values correct is far more important than having good shadow detail. Especially when its dark. And besides, I think you will find there is more shadow detail than you might expect.
    Not a scientific curve log log parallel line reference old theoretical article approach I know, but it works without ruining highlight contrast. And especially for roll film users who can't develop each frame independantly.
     
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  15. Nicholas Lindan

    Nicholas Lindan Advertiser Advertiser

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    I agree, this is the right way to do it if you can't adjust the development time.
     
  16. RobC

    RobC Member

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    I question this because if we can see detail in a dark area at night, then we think that should be a zone II or III. However, if you then meter a highlight and place it where it falls, it may be off the scale and more so after applying reciprocity. Whereas, if you meter a highlight first and place it on zone VII or VIII and then meter the shadow area, you will likely find its zone 0 or below. So the question is, how is it that we can have one way of metering saying shadow is going to be zone II and another saying its going to be zone 0 or less. The answer is that our judgment about where a shadow should be at night is fooled by the fact that we THINK it should be zone II or III but metering for the highlights tells us it is really at 0 or below. If we go with the meter for shadow approach you are effectively trying to turn a night shot into a daylight shot by overexposing the shadows with the resultant bad effect on the highlights because of the reciprocity factor which is applied. People forget their eyes are pretty good at opening up to see shadows at night and also closing down to see highlights when they are viewed after looking into the shadows. If the scene has high contrast as metered then using the highlights for metering will retain the look of the scene more closely than will applying dogma to rendering shadow detail.

    All the theory about reciprocity curves does not take that into consideration. And why should it, reciprocity is about films responsivness and not about where the values should be placed. And where the values should be placed is about making the correct choice for conditions and night time/low light conditions are rendered better by metering for the highlights because it removes the dogma about shadow detail which I beleive is not valid if you want a night time shot to look like night time and not day time.
     
  17. gainer

    gainer Subscriber

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    You will run into this situation long before the shadows become too dark to see, and certainly before they become too dark to measure with a modern meter. When you cannot see the shadow you want for max black, don't even bother measuring and/or calculating. When Bond did his experiments, he had to be able to measure light levels low enough to require total exposures in the neighborhood of 400 seconds. Don't forget, when you use a view camera and f/64 is where its at, 500 seconds in a cave or a medieval cathedral is not out of the picture, literally. Your comment about more shadow detail than you might expect is agrees with what Bond said. He did not find much change in gradation. That means that whether you measured his situations for the films he tested using any point along a normal characteristic curve, the same exposure increment applied to the meter calculation of exposure would suffice. Let me say that a different way. If I take a reading with a good spot meter of a scene brightness that I want to be in Zone I or II or V and calculate the exposure I would use in order to place that brightness in that Zone as if there were no reciprocity misbehavior, I would add the reciprocity to the calculated exposure time. That is why you should use the experimental data for the same Zone for all points on the correction curve or table. If you read a different Zone in an actual scene because you cannot see into the Zone that was used in forming the correction chart, you simply add into the correction the difference in exposure time between the Zone you measured and the Zone you were able to see. Just because you cannot see it doesn't mean the camera cannot. Now, of course, it becomes necessary to make some estimate of CI that you must use. Bond said that unless you know more than he learned in those experiments, the best bet is to develop as you would normally and depend on paper grade, dodging and burning, or MG filter to make up the difference. If you are going to tell me that you never use such trickery...I'll tell your Mommy.

    The idea of the Zone system is to be able to position an arbitrary Zone on the characteristic curve so that tha curve will show all Zones from I to X so that the paper can show them, and it is designed to do so whether or not you can see all the Zones. If you can see one and know the available paper density range, you can do it.
     
  18. RobC

    RobC Member

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    Well almost, but the point I have been trying to make is that making the correct judgement in the dark is very diificult and for most night time shots, using a highlight value will do it for you. As for the extreme case of a cave, if it were that dark, then there will be little contrast in the shot so you will be needing serious additional development to boost contrast.
    However, take a night time shot in a town with a dark alley but also some illuminated building. Meter from the dark alley which you can just see and you may get say 5 seconds for zone III which apparently needs reciprocity correction. However, meter for the illuminated building on a zone VII and it doesn't need any reciprocity correction. The two points are in different lighting. So which do you expose for because what applies to one point doesn't apply to the other point so your graph projections are as much use as a chocolate kettle. You have to pick one lighting source over the other and what I am saying is, if you expose for the highlight then most of the time you will get a far better rendition than if you expose for a shadow reading which required more reciprocity correction.
     
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  19. gainer

    gainer Subscriber

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    That is exactly the point I was trying to make. You make the reading that you can see. Let's say it happens to be the brightness you would like to see in Zone VI. Do you place it in Zone X on the paper? Not likely. You want some headroom for accents, etc. It doesn't matter if you can see the lower and higher Zones or not, you can still expose that brightness as if it were Zone VI. After all, the other values of SBR may be in areas too small to get a proper reading with the meter at hand. We do such things often. If we spy an area that looks to be the same brightness we're concerned with, we substitute it for the meter reading. Or we read something we judge to be a certain number of Zones higher or lower but is large enough to be measurable. Now why must we change that part of our technique just because the light is too dim to allow exposing the precise area we want to be in a certain Zone? What other mystery does reciprocity hold for us than requiring more exposure time? Is there some different time that must be added to each brightness? If there is, there is no way we can do it. At least not without resorting to the evil of digital computation and a digitally controlled scanning device in place of the shutter. That is why the film speed, CI, Zone or related systams, etc were invented and are used. Now we forget all that because we are worried "What if the characteristic curve changes with long exposure times?" I contend that if you know the Zone you want to use to print whatever part of the scene you can see, you can use the exposure you would normally calculate with your meter for that Zone I and f/stop, and add the correction for reciprocity to that time. If you do learn that for a given film the CI for given development varies with exposure time, you cannot change that by changing exposure time. It can only be changed by development.
     
  20. spoolman

    spoolman Subscriber

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    B&W film reciprocity calcs.

    Hi Ray:I shoot buses,streetcars and trains at night and I find that my usual films of choice,plus-x,t-max 400 1st.edition and hp5+ seem to have either not enough contrast,as with plus-x or too much contrast as with hp5+ or tmax 400 1st. ed.I've tried adjusting development times and dilutions but nothing seems to give me what I'm looking for.Relatively even contrast,with detail in the shoadows and highlights.My developer of choice is Rodinal for plus-x@1:50 dilution,same for tmax400,and D-76 1:1 for hp5+.

    Any suggestions?.

    Doug
     
  21. spoolman

    spoolman Subscriber

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    B&W film reciprocity calcs.

    Hi Rob: Thanks for this info.It's a simple and understandable solution to this problem.Next time I'm shooting at night.I'll do a test roll and see how it turns out

    Doug
     
  22. spoolman

    spoolman Subscriber

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    B&W film reciprocity calcs.

    Hello Ian:Thanks for the suggestion.I'll get to downloading the sheets tomorrow.Didn't it used to be simpler when film manufacturers supplied such info with the film !!!.

    Doug
     
  23. spoolman

    spoolman Subscriber

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    B&W film reciprocity calcs.

    Hello:To all who replied to my question regarding B&W film reciprocity calcs.,Thank you for all your insights.I will try each method and suggestion when I have time.

    Doug:smile:
     
  24. RobC

    RobC Member

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    Good Luck with it. If you find using this method that your shadows are blocked up too much, you will need a softer negative in which case reduce film speed by 2/3 stop(to give extra exposure) and also reduce dev time by 30% (to give less film contrast) and try again. But I bet that the first time you try it, you will nail the highlights.
     
  25. Murray@uptowngallery

    Murray@uptowngallery Member

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    I went looking for the mkaz site. Thanks for re-listing it.

    There was also a mkirwan site with some collected data. I think it included the Gainer-Bond data in a graph or spreadsheet, possibly additional film data, but couldn't get to it (dead link).

    Anyone know if he still has a site?

    Thanks

    Murray
     
  26. fschifano

    fschifano Member

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    I'd say right off the bat, that Rodinal is probably at least partially to blame. It's not very good at holding shadow detail, and you are working under challenging conditions. I've always been more than happy with Plus-X and D-76. XTOL will help even more with the shadow details and will hold the highlights in check. I find that this is even more applicable to TMax 400. Since you are already using D-76, give that a try next time and see if you don't get an improvement with these two films. Next time you're restocking chemistry, pick up some XTOL and give it a go. It works very well with HP5+ too.