Originally Posted by jeroldharter
Thanks for the word of confidence, but my last post was unfortunately incorrect. Your post mentioning speed 'loss' got me a bit off track (my mistake). The speed curve dipping into the negative means that less exposure was needed to get the same print density, of course. Therefore, we are talking about a speed 'gain' with time. This happens for the print highlights but to a lesser degree to the shadows, which explains the extended log exposure range, meaning loss of contrast. It's a bit like a fogging exposure, if you know what I mean.
I remember discussing this with Ilford at the time. I'm not sure that the underlying effect is clearly understood. It was explained to me as 'propagating' exposure.
Sorry for the confusion, I had it right in the book manual (sigh of relief).
Ralph, I'll also be looking forward to getting a copy of the book. Wish I had known about it years ago!
In the meantime, I can add another twist to your method; I'm something of a veteran of latent image testing.
When making your test exposures, with a long time between first and last, it's possible that your exposing device (enlarger, in this case) has shifted; that is, the exposure right now vs the exposure one hour ago are different. If you want to confirm there is no change, this turns out to be very difficult to do. One might suggest to make all of the exposures at once, then process periodically. In this case, the possibility exists that the processing condition has changed somewhere between start and finish of the test.
A way around these possible shifts is to expose everything at the same time, and later process everything at the same time. But how do we get the latent image shift? Well, it turns out that freezer-temperature storage will virtually halt latent image shift. Yes, you can do more tests to confirm this, but it's a reasonably safe assumption with conventional materials.
So, the entire procedure is this: expose some test strips, perhaps 30, for example. Randomly pull out about half, for evaluation of consistency of your exposure and processing. People who've done statistical process control will appreciate the point of this. Quickly get the remaining samples into a freezer, obviously(?) light-tight water-proof containers are used. Then, at intervals, remove samples from the cold storage. Finally,process all samples together.
The evaluation seems relatively straightforward. I think it goes without saying, but let me say it anyway, if the "control group" of prints has more variation than the "latent image test" group, then nothing is known about the latent image shift, except that it is less than the normal "noise" in your process.
The usual result is that the greatest changes happen fairly quickly, then the rate of change slows down more and more.
ps; I wouldn't test this far in my own darkroom, but in industry, etc, it's a good way to go.
I started a test about three hours ago. The protocol is -
- 6 5x7 sheets of paper were contact printed with a step tablet over half their surface.
- The sheets are stored in their black plastic bag along with a small stock of unused paper.
- At intervals, the other half of the sheet is contacted with the same step tablet with the same exposure. The light source is stable and closed-loop.
- The sheets are developed promptly after the second exposure.
- Any difference between the two step tablet images on the same sheet should be due to latency effects, as the two images are developed at the same time.
So far sheets with 1/2 hour and three hours of latency have shown no visible change. Sheets from the all three (0, 0.5 and 3.0 hour latency) exposures show no sheet to sheet differences so developer effects don't seem to be coming into play, though aged developer is first apparent in shadow density and not in the highlights, whereas the latentcy-fog is in the highlights.
The next sheets will be at 16 hours and then again at 48 hours.
I have, however, noticed the effect Ralph mentioned - a sort of self-evolving fog. I had an Jobo LED safelight shining over the paper trimmer. The safelight passed all fogging tests. However, when I used paper that had been trimmed under the safelight, it exhibited fog if it was used after a few days of sitting loose in the paper safe. After changing the safelight to a Kodak OC filtered light there is no more 'trimmer fog'.
This whole thing is getting curiouser and curiouser... It is possible that the storage conditions - as regards exposure to air between exposure and development - may have some bearing.
Last edited by Nicholas Lindan; 09-02-2009 at 11:47 PM. Click to view previous post history.
Nicholas, it sounds like a well arranged test. My only concern would be with respect to stability of your light source, but I'm not real knowledgeable about the electronic control systems. My feeling is that you are, so I presume it's a solid system.
That speed gain mechanism seems very odd. I could have used that on film, back in my younger days, when I was pushing film as much as I could for available light shots.
On with the test results -
The 3 hour latency print is now dry, and shows a definite increase in the highlight speed of the paper, clearly visible to the eye.
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Dear all. I can't saying - thank you for answers and knowledge.
Originally Posted by Mr Bill
I was fortunate enough to receive 6-sigma black-belt training recently, so, I understand your comment. It makes a lot of sense to do the test this way.
My test was done (thank God for lab records) by exposing the first strip and timing the time to processing (8s). Then, doing the same for the next few (16s, 32s, 60s, 2min, 4min), after which the rest was exposed, one after the other, and stored in a light-tight drawer, where they 'waited' until it was their time for processing.
My L1200 is hooked up to a voltage stabilizer and the exposures are very consistent. Dektol 1+2 in a covered tray does not show any degradation within 4 hours and my basement darkroom temperature is also very consistent unless I remain in it, which will warm it up, but I did not do that in this case.
Nevertheless, your recommendations are very valuable indeed.
Originally Posted by Nicholas Lindan
It's very odd isn't it? But, I like that we are performing global darkroom testing at APUG now. This is all very interesting.
Storage conditions of the exposed paper are important. As I said above, my test strips where stored in a light-tight drawer until they were ready for development, which was opened, of course, to take out the next piece of paper. However, since they were sitting on top of each other, every test strip got the same few additional seconds of safelight exposure.
How did you manage to minimize additional safelight or processing light to get to the aged test strip while doing a second exposure on the same piece of paper?
Sorry for robbing your thread.
Originally Posted by jongcelebes
However, you have sparked an interesting discussion and test series. The answer must have been more than you expected?
I mean, I can't say anything but thank you. I get info more than I need (and it's good).
This forums make people like me (no access to workshop/teacher) to taste darkroom experience. Even it might brutally wrong and unusual.