Latent Image Stability
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
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
30 sec 89
1 hour 85
2 hours 78
4 hours 75
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?
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.
Slow films in particular are known to have poor latent image stability... Old Pan-F+ and Tech Pan show faint or lost rebate markings.
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.
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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.
Makes more sense to me now. I should have read the table more carefully and realized 30 seconds was unreasonable. Anyhow, very interesting post.
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"
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?
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.