Latent Image Stability

Discussion in 'B&W: Film, Paper, Chemistry' started by Stephen Benskin, May 30, 2011.

  1. Stephen Benskin

    Stephen Benskin Member

    Messages:
    1,646
    Joined:
    Jan 7, 2005
    Location:
    Los Angeles
    Shooter:
    4x5 Format
    I found this while going through some old test results. It is a latent image stability test. After the film is exposed and before it is developed, silver atoms, particularly in low exposure areas, have a tendency to lose electrons and revert back to an undevelopable state. This has a tendency to lower film speeds but also has an affect on overall contrast. Most of this loss happens within moments of exposure and then plateaus off over time. Storage temperature of the exposure film can play a part of the rate of the regression of the latent image.

    The ISO speed standards have a "hold time" incorporated into the standards. It was originally two hours, but with ISO 6 it changed to 4 hours to 7 days for professional B&W film and 5 to 10 days for general purpose film to better reflect real world usage.

    The test I conducted exposed sheets of 4x5 film over the course of a month using a calibrated EG&G Mark VII Sensitometer. All the film was processed at the same time.

    Latent Image Keeping Test
    Hold Time EFS
    Test 1
    30 sec 94
    1 hour 78
    2 hours 78
    3 hours 78
    4 hours 78
    1 day 74
    2 days 71
    5 days 71
    7 days 71
    14 days 68
    21 days 68
    31 days 65

    Test 2
    30 sec 89
    1 hour 85
    2 hours 78
    4 hours 75
     

    Attached Files:

  2. michael_r

    michael_r Subscriber

    Messages:
    6,539
    Joined:
    Feb 2, 2010
    Location:
    Montreal, Canada
    Shooter:
    Multi Format
    Stephen, can you explain that the data shows. I'm not sure how to interpret it. Eg: for 31 days in Test 1, what is the 65? Also what is the different between Tests 1 and 2? What film is this?

    Thanks
    Michael
     
  3. Stephen Benskin

    Stephen Benskin Member

    Messages:
    1,646
    Joined:
    Jan 7, 2005
    Location:
    Los Angeles
    Shooter:
    4x5 Format
    Column 2 represents the effective film speed. The post didn't maintain the formatting. I included an attachment to help clarify.

    I'm concerned that if I disclose the film type, the test would become more about the specific film and not about the general concept of latent image stability.
     
  4. bwrules

    bwrules Member

    Messages:
    198
    Joined:
    Jun 18, 2008
    Shooter:
    Multi Format
    Slow films in particular are known to have poor latent image stability... Old Pan-F+ and Tech Pan show faint or lost rebate markings.
     
  5. michael_r

    michael_r Subscriber

    Messages:
    6,539
    Joined:
    Feb 2, 2010
    Location:
    Montreal, Canada
    Shooter:
    Multi Format
    Stephen, I guess I'm just surprised the loss is so great after one month so I was curious to know whether different films can behave very differently regarding latent image keeping. Losing 2/3 stop in one month is pretty major.
     
  6. Stephen Benskin

    Stephen Benskin Member

    Messages:
    1,646
    Joined:
    Jan 7, 2005
    Location:
    Los Angeles
    Shooter:
    4x5 Format
    You can't think of it in those terms. The film didn't really have a film speed of 98 because who develops the film within a minute of exposure? As hold times should be longer than two hours for film speed testing, the speed is actually more like 78. Hold time is a frequently overlooked variable in film speed testing.

    Film would be even faster if it were possible to process within fractions of a second. Reciprocity failure is actually a form of latent image regression.

    Traditional fine grain films have more uniform grains. As a group, they tend to either be able to be developed or not. That's why they tended toward higher contrast compared to the same levels of development as faster films, and why litho films are very fine and uniformly grained.
     
  7. michael_r

    michael_r Subscriber

    Messages:
    6,539
    Joined:
    Feb 2, 2010
    Location:
    Montreal, Canada
    Shooter:
    Multi Format
    Makes more sense to me now. I should have read the table more carefully and realized 30 seconds was unreasonable. Anyhow, very interesting post.
     
  8. ic-racer

    ic-racer Member

    Messages:
    7,473
    Joined:
    Feb 25, 2007
    Location:
    Midwest USA
    Shooter:
    Multi Format
    Thanks for posting that.
    Anything else in the ISO specs to be aware of. I suspect the spectrum of the EG&G is compatible. How about exposure duration, are you using 1/1000? How do they specify the developer, brand name or composition? Is there an allowable deviation or +/- range in the standard?

    How much do you think this affects gamma? For a routine development time control strip, OK to fire off the test strip and process right away?

    Last and OFF TOPIC: You are one of the few here that has sent their EG&G to be measured/calibrated. When you do an ISO test and the results differ from the box speed what is your response? Calibration off? Error in testing conditions? Bad film? or "I wouldn't even try to exactly duplicate ISO conditions"
     
  9. RalphLambrecht

    RalphLambrecht Subscriber

    Messages:
    8,213
    Joined:
    Sep 19, 2003
    Location:
    Florida
    Shooter:
    Multi Format
    Stephen

    Very interesting! First, I was surprised to see this rapid of a loss, but thinking about it (and apart from 30 s), it's not that big of a deal. My latent image requirement for is usually around a few hours up to a day or two.
    Do you still have the transmission density data for this test? Would be interesting to see how much density individual densities lost. How did Zone I compare to Zone V for example?
     
  10. Allen Friday

    Allen Friday Member

    Messages:
    877
    Joined:
    Mar 30, 2005
    Shooter:
    ULarge Format
    My take away from this: If you normally develop your film within 1 to 4 days after shooting, do formal tests 1 to 4 days after exposing the test film. This will give you an EI that will work for real world use. If you are going on a long trip and can't develop film for a month, you may have to give a slight boost to exposure (or corresponding drop in film speed). But, a drop from EI 71 to EI 65 is within my margin of error.

    If you don't want to wait to do tests, (such as with BTZS tests where you expose film under the enlarger and can move directly to development), then be aware your EI may be a bit generous. But, you should be making the necessary adjustment anyway following the last step in BTZS testing--Field Testing.
     
  11. frobozz

    frobozz Subscriber

    Messages:
    1,362
    Joined:
    Feb 19, 2010
    Location:
    Mundelein, I
    Shooter:
    35mm
    OK, that brings up a question I had: I recently started playing with some old supposedly well kept 35mm Tech Pan from a factory sealed 150 ft roll. There are no rebate markings of any kind. I assumed maybe there just weren't supposed to be any, but now I wonder....

    Duncan
     
  12. RalphLambrecht

    RalphLambrecht Subscriber

    Messages:
    8,213
    Joined:
    Sep 19, 2003
    Location:
    Florida
    Shooter:
    Multi Format
    And then some. A drop from EI 71 to 65 is a bit more than 1/10 stop. My margin of error (considering measurement, shutter and aperture inaccuracies, and all processing errors) is far higher! I'm glad to be within 1/3 stop.
     
  13. Stephen Benskin

    Stephen Benskin Member

    Messages:
    1,646
    Joined:
    Jan 7, 2005
    Location:
    Los Angeles
    Shooter:
    4x5 Format
    Many of the changes to ISO 6 was to bring it more in line more with real world usage. It eliminated the ISO developer. Some might remember that TMX and TMY didn't have ISOs when they were first released. T-grain films didn't do well under the ISO developer. Kodak tested the Tmax films using D-76. Kodak couldn't release the Tmax films with ISO speeds because the speeds resulting from the ISO developer wouldn't represent the results obtained from most general purpose developers. I believe this motivated the change in the standard. According to ISO 6, manufacturers can use any developer and any method of development they want. They are supposed to note it in accompanying literature (which most don't).

    The EG&G has is balanced to daylight. Interestingly, the 1960 standard changed from sunlight to daylight which changed the film speed constant. Under sunlight, it would have been 1 as in 1/Hm, but the increased level of blue with daylight caused the constant to be adjusted to 0.8 (0.8/Hm). This is a perfect example of how controlled scientific testing conditions can produce the same results as from real world usage.

    I just double checked ISO 6 and it now says that speeds "may be determined using ISO sensitometric daylight, studio tungsten, or photoflood illuminants. Since the speed of film/process combinations will depend on the type of illuminant used for determining ISO speed, the illuminant should be specified in use instructions." Good luck finding those.

    I always use the 10^-4 setting because 1/1000 sec is within a regular range of use and the illumination was low enough. For some reason EG&G had 3440 mcs at 1/100 sec and 59.4 mcs at 1/1000 sec. I would then use filters to bring the illuminance down to a more usable range. The ISO standard has it as between 5 sec and 1/1000 sec.

    Temperature is something many don't consider. Latent image regression increases with higher temperatures. That's why you are supposed to store control strips in the freezer or fridge. The standard has the test to be held at 23 degrees C +- 2 degrees at a relative humidity of 50% +- 5%.

    The film gradient is supposed to also drop some. With the lab work, I would take the Kodak control strip out of the freezer and hold it for one hour before processing. With the sensitometric tests, I would hold them for two hours after exposing them.

    To answer Ralph's question about keeping the test data from the latent image test, I wish I did. That was before my programs and I did all paper plots. It was easy to misplace them or simply throw them out because they were talking up too much room. The beauty of the developing part of that test is that I was using a Refrema dip-and-dunk which held 10 sheets. I loaded nine sheets and just before it took the rack, I exposure the 30 second sheet and literally loaded it while the rack was being picked up. It's not something one can do with a Jobo.

    What is important to take away from this test is to have a reasonable hold time and be consistent with in in your testing if you want consistent results. Being aware of the need for a hold time probably comes first though (something Adams never mentioned).

    When I did it for the labs, we were processing in seasoned chemistry. I never expected the speeds to be optimum. But if I had a question, I would retest. It wasn't my money so I could be as anal as I wanted. Everyday, I'd run a morning Kodak control strip and a sensitometric test on 4x5 TMX. Control strips were made on TMY. Different films will react differently to changes. I'd see the control strip's speed drop while the TMX remained consistent. The TMX "wedge" could also be used to gauge consistency of the sensitometer's exposure. At one point when we were running tons of film, the wedges were dead on the same day to day.

    One interesting example came from HP5-Plus in T-Max RS. The RS was rather active and would achieve normal (CI 0.58) at around 3 minutes. The very short development time would produce effective film speeds of around 100. For unexpected results like those, I can tell you there was more than a single retest involved. This finding eventually lead to modifying the RS and then ditching it the moment Xtol came on the scene.

    ISO conditions aren't that far off from normal conditions except for the precision in exposure in order to get knowable and repeatable exposures. Believe it or not, the EG&G sensitometer doesn't conform to the ISO standards. It has an intermittent system for the exposure (flash). ISO states a non-intermittent system. I once had a yelling match with some jerk at Kodak over the precision of an intermittent system.

    From Ralph
    I like to take these results and apply them conceptually to reciprocity failure. With long, low energy exposures, you can imagine the loss of speed as the latent image in the shadows keeps regressing before receiving sufficient exposure to stabilize it.

    As Ralph can tell you, that is why film is rounded to the nearest 1/3 stop and ISO film speeds aren't determined from a single test, but from multiple tests over a period of months using film from a number of batches. The biggest problem when using a calibrated sensitometer and a small sample population is when the speeds fall close to the rounding point - an 89 would round to 80 and 91 would round to 100. With a densitometer only as accurate as +- 0.01, it can become anyone's guess in any one test.
     
  14. Sponsored Ad
  15. Photo Engineer

    Photo Engineer Subscriber

    Messages:
    25,774
    Joined:
    Apr 19, 2005
    Location:
    Rochester, NY
    Shooter:
    Multi Format
    Most all current films contain from 2 - 3 emulsions. Each emulsion responds to LIK differently and therefore the contrast changes and the speed changes individually.

    The true measure is not an ISO "point" but rather the entire curve. Any other measure is misleading to say the least.

    If the speed point changes, but the mid scale stays the same, can we say that the film has changed speed? This is where the image goes, so what can be said, must be said about the actual image, the curve if you will!

    PE
     
  16. Stephen Benskin

    Stephen Benskin Member

    Messages:
    1,646
    Joined:
    Jan 7, 2005
    Location:
    Los Angeles
    Shooter:
    4x5 Format
    I believe we can. The relationship between the speed point and the metered exposure or k1 is 10. By defining the speed point as 0.8/Hm, the metered exposure will fall at 8/ISO. What density this is isn't part the film speed / exposure meter relationship. So, if the mid scale stays the same, it isn't a factor in determining film speed. The loss of density is proportionally greater in the areas low exposure so in effect even without dealing with multiple emulsions, the speed point will generally change and the mid scale stays close to the same anyway. So yes, the film speed has changed. This is how the ISO standard defines film speed (ISO 6 1993), and I don't believe there has been a need to update it. It defines the hold time for a reason.

    The purpose of the original post is to illustrate the importance of a hold time in testing to eliminate possible misleading and inconsistent results.
     
  17. Photo Engineer

    Photo Engineer Subscriber

    Messages:
    25,774
    Joined:
    Apr 19, 2005
    Location:
    Rochester, NY
    Shooter:
    Multi Format
    We might say OTOH, that the toe contrast has changed. Or we might say that the fast component has changed if the other parts of the curve remain unchanged. In either case, I would change the fast component in some if this is a problem. The only way to determine if it is a problem is to check it under normal use.

    Basically though, a single point does not define LIK in the same manner as a single point does not define reciprocity.

    PE
     
  18. Stephen Benskin

    Stephen Benskin Member

    Messages:
    1,646
    Joined:
    Jan 7, 2005
    Location:
    Los Angeles
    Shooter:
    4x5 Format
    To this I agree.
     
  19. RalphLambrecht

    RalphLambrecht Subscriber

    Messages:
    8,213
    Joined:
    Sep 19, 2003
    Location:
    Florida
    Shooter:
    Multi Format
    Stephen

    Can you check if you still have the density values from your test?
     
  20. Stephen Benskin

    Stephen Benskin Member

    Messages:
    1,646
    Joined:
    Jan 7, 2005
    Location:
    Los Angeles
    Shooter:
    4x5 Format
    Ralph,

    I believe they are long gone. Those were hand drawn from a time before I began writing the plotting programs.
     
  21. michael_r

    michael_r Subscriber

    Messages:
    6,539
    Joined:
    Feb 2, 2010
    Location:
    Montreal, Canada
    Shooter:
    Multi Format
    Stephen, what is your photographic and/or scientific background. I remember seeing your name referenced in the Film Developing Cookbook (it might have been the Darkroom Cookbook, I can't remember). Sorry if it's a dumb question, I was just curious. :smile: It occurred to me the other day to try (mostly through mindless name "Googling") to find out more about some of the people on here who have been helpful to me over the past year as I started re-testing/calibrating my materials, people like yourself, Ian Grant, Ralph Lambrecht and Ron Mowery.
     
  22. RalphLambrecht

    RalphLambrecht Subscriber

    Messages:
    8,213
    Joined:
    Sep 19, 2003
    Location:
    Florida
    Shooter:
    Multi Format
    Oh, my God! Now my reputation is completely ruined. I wonder if they let me back into the US tomorrow.
     
  23. michael_r

    michael_r Subscriber

    Messages:
    6,539
    Joined:
    Feb 2, 2010
    Location:
    Montreal, Canada
    Shooter:
    Multi Format
    At least I can see some of your work on your website. Stephen is a much more nebulous character :smile:
     
  24. RalphLambrecht

    RalphLambrecht Subscriber

    Messages:
    8,213
    Joined:
    Sep 19, 2003
    Location:
    Florida
    Shooter:
    Multi Format
    That's easy to explain. He spends too much time around Zone I. I'm more comfortable around Zone VII.
     
  25. michael_r

    michael_r Subscriber

    Messages:
    6,539
    Joined:
    Feb 2, 2010
    Location:
    Montreal, Canada
    Shooter:
    Multi Format
    Makes sense to me!
     
  26. Stephen Benskin

    Stephen Benskin Member

    Messages:
    1,646
    Joined:
    Jan 7, 2005
    Location:
    Los Angeles
    Shooter:
    4x5 Format
    I don't spend time around Zone I. Remember Ralph, I'm a Zone buster, destroyer of Zones. Well, more like attempting to replace imprecise Zone System terminology and misconceptions with sensitometry but in a heroic way in my mind...and without a cape.
     
    Last edited by a moderator: Jun 1, 2011