Curve of the fading of the latent image in modern emulsions? And the possible speed gain by developing really quickly?

Helge · Feb 18, 2023

You read the title. So how about it?
Latent image fades much faster in the first minutes and hour after exposure, until it reaches a very slightly slanted plateau. A plateau that stays ≈ level for months and even decades esp. with freezing.

But, what if you developed the film very quickly after exposure? As in minutes after. How many potential stops to be gained? One? Two?
What would the difference be between fast and slow film?

The obvious answer would be “experiment”. But I don’t have the time or inclination right now. I’m looking for someone who has done the experiments or can point me in the direction of som data or articles.

aparat · Feb 18, 2023

Good question. This is known as the "latent image regression," and I remember it also being called "the Christmas tree effect," but that is not important. The important thing is that, as you noted, the first few hours are critical, after which the image is relatively stable. Yes, it does degrade over time, but less so. Color film is more prone to this effect than B&W.

Having said that, the effect is really minor (within a reasonable time), and is about of the same degree as batch-to-batch film stock variability, sensitivity limits of your instruments, and, of course, stability of one's processing system. However, for critical work, such as sensitometric testing, one should try to process the film immediately after exposure.

I have found this plot of the curve. It's a theoretical curve but it shows the general effect (Photographic Materials and Processes, Stroebel, Compton, Current, Zakia, Focal Press, 1986, p. 282).

Doremus Scudder · Feb 18, 2023

aparat,

Your chart shows the effect of age fogging on film, and rather generally at that. The difference in the curves at the low end is attributable to fogging, not latent image degradation, which is a different thing entirely.

FWIW, I've done the "develop right away vs developing a week later" test once or twice and really didn't see a whole lot of difference in the resulting negatives. That said, I usually give a rather "full" exposure, so even a bit of effective speed loss may not have affected my shadow values that much. Contrast was comparable. That's with the ~ ISO 320-400 speed films I usually use; results may be different with slower films. And, I didn't do any real scientific measurements, just kind of eyeballed the negs.

Best,

Doremus

brbo · Feb 18, 2023

A few times I shot film and developed it within minutes. I never observed anything out of the ordinary. Of course, I never shot a second shot to develop it a week later to have true comparison to the negative developed within minutes. But exposures came out pretty much as I expected. Certainly nothing even close to a stop or two boost...

For films I haven't seen any mention for minimum time between exposure and development, but for RA-4 paper Kodak says that latent image shouldn't change noticeably within 5s and 24h after exposure.

ic-racer · Feb 18, 2023

I don't have a curve, per say, but you could make one from this data:

The table shows, for example, a mid value (0.5 h Point) can lose one-half stop (0.15 log E) if held in an 80% humid 20% oxygen/70% nitrogen environment for 72 hours.

Likewise, film held in a vacuum for 72 hours can gain 1/6 stop (0.05 Log E).

IS&T’s 1999 PICS Conference
The Effect of Environment on Latent Image Formation and Stability
Sean W. P. O’Toole Eastman Kodak Company Rochester New Y ork

ic-racer · Feb 18, 2023

ISO specifications for B&W film handling when testing speed:

aparat · Feb 18, 2023

Doremus Scudder said:
aparat,

Your chart shows the effect of age fogging on film, and rather generally at that. The difference in the curves at the low end is attributable to fogging, not latent image degradation, which is a different thing entirely.

It is labeled in such a way that it can be misleading. As I mentioned, the curve is theoretical and only shows the "gross" effect for comparison's sake. Here's the quote that accompanies the plot:

"Since the latent image consists of a cluster of silver atoms, the loss of only a few atoms may render a silver halide grain undevelopable, An atom of silver can combine with an atom of bromine, for example, to form a molecule of silver bromide - reversing the effect of exposure. After long periods of time the image, when developed, may have less contrast and be less distinct due to the spontaneous development of a large number of unexposed silver halide grains. As this type of development fog increases, a point is reached where the developed latent image can no longer be detected. A small amount of development fog, however, can produce the effect of increasing the speed of photographic emulsion and the density of the developed latent image in the same manner as latensification."

Rudeofus · Feb 18, 2023

Here is a thorough test done by Stephen Benskin on photrio.

Bill Burk · Feb 18, 2023

ic-racer said:
ISO specifications for B&W film handling when testing speed:

View attachment 329976

Thanks, I have been looking for that. I knew a hold time is required for this specific reason.

Maris · Feb 18, 2023

Which prompts this conjecture:
Has anybody soaked unexposed film in developer until it was saturated and then loaded it into a wet plate holder for in-camera exposure? By this method there would no time interval between exposure and development and thus potentially achieving maximum possible film speed.

ic-racer · Feb 18, 2023

The results quoted in post #5 indicate the film was fastest 3 days after exposure [when stored in vacuum].

Mr Bill · Feb 18, 2023

Helge said:
But, what if you developed the film very quickly after exposure? As in minutes after. How many potential stops to be gained? One? Two?

Hi, I'm from an outfit where we actually did latent image shift testing on materials that we used (or considered for use). But... our test period was fairly coarse, with the shortest interval (expose-to-develop) on the order of a half-hour to an hour. And, as I recall, we ran tests out to only about 5 days (maybe to a couple weeks once or twice).

Now, this testing was very limited, being done for a specific purpose. As part of a large portrait chain, processing film in a central lab (or in several "satellite" labs) we wanted to know two things. First, was there any significant change in the film during the maximum time delay of perhaps a week, and second, how long must we hold exposed film before processing for internal tests? Also, the test materials were mostly limited to Kodak's low-speed professional portrait/wedding color neg films over the years - specifically VPSII, VPSIII, and the original Portra 160 NC (all C-41 films).

The results: well, my last direct involvement was near 30 years ago, so this is from a shaky memory. We evaluated from full sensi curves, not from any specific calculations. There WERE some slight differences between one and fours, but very little after that. So we concluded that, for fairly critical testing, it was adequate to hold exposed film, under general controlled office conditions (70-75 F, ~50% RH) for 4 hours or more.

Probably what people here would be more interested in would be, how much change between 1 and 4 hours hold time? I really don't recall, except that it was not all that large. We did not specifically try to quantity it as this was not something we cared about - our interest was: how long to hold before processing, to be representative of production film up to a week old. If I HAD to make a guess, from old memory, I'd say perhaps similar to the effect of 2/10 to 3/10 degree F error in C-41 developer temperature. But remember, this is from a shaky recollection of something that we did not specifically evaluate.

Fwiw I would NOT automatically presume that these results are representative of all other films; these particular films were premium professional products, and latent image shifts could cause a color problem for those particular customers. So obviously the manufacturer pays special attention to this.

Mr Bill · Feb 18, 2023

ic-racer said:
I don't have a curve, per say, but you could make one from this data:

Hi, I don't know if you have a copy of the full paper for this, but if not, it's worth pointing out that the table you are using was for a hand-made emulsion that was specifically not sensitized. So it's very unlikely to behave like a real-world emulsion.

In fact, in the Abstract the authors say, "The unsensitized emulsion was the most sensitive to environment while the sulfur-plus-gold-sennsitized was not."

As a long-time on and off member of the IS&T I learned that one has to be careful interpreting these papers.

RalphLambrecht · Feb 19, 2023

aparat said:
Good question. This is known as the "latent image regression," and I remember it also being called "the Christmas tree effect," but that is not important. The important thing is that, as you noted, the first few hours are critical, after which the image is relatively stable. Yes, it does degrade over time, but less so. Color film is more prone to this effect than B&W.

Having said that, the effect is really minor (within a reasonable time), and is about of the same degree as batch-to-batch film stock variability, sensitivity limits of your instruments, and, of course, stability of one's processing system. However, for critical work, such as sensitometric testing, one should try to process the film immediately after exposure.

I have found this plot of the curve. It's a theoretical curve but it shows the general effect (Photographic Materials and Processes, Stroebel, Compton, Current, Zakia, Focal Press, 1986, p. 282).

View attachment 329969

I don't think your plot is on latent image regression put on film age. To compel the required by the OP it will take a lot of film in a lot of time; not sure anybody would invest there knowing that development should be done as soon as possible.

Rudeofus · Feb 19, 2023

Mr Bill said:
Probably what people here would be more interested in would be, how much change between 1 and 4 hours hold time? I really don't recall, except that it was not all that large.

It may not seem large to you, but from the data I have seen (and linked to here) the difference is in the same league as all these "speed enhancing developers", which often create quite a bit of extra grain. But yes, in real life, one half stop is rarely decisive.

aparat · Feb 19, 2023

RalphLambrecht said:
I don't think your plot is on latent image regression put on film age. To compel the required by the OP it will take a lot of film in a lot of time; not sure anybody would invest there knowing that development should be done as soon as possible.

No, it's not. As I explained later in the thread, the authors of the book used that plot to illustrate the general effect of the reduction of contrast and build up of fog. The labeling is unfortunate, I agree. I also agree that to test current emulsions in current developers would take a lot of work.

ic-racer · Feb 19, 2023

Mr Bill said:
Hi, I don't know if you have a copy of the full paper for this, but if not, it's worth pointing out that the table you are using was for a hand-made emulsion that was specifically not sensitized. So it's very unlikely to behave like a real-world emulsion.

In fact, in the Abstract the authors say, "The unsensitized emulsion was the most sensitive to environment while the sulfur-plus-gold-sennsitized was not."

As a long-time on and off member of the IS&T I learned that one has to be careful interpreting these papers.

Thank you for the clarification.

I believe commercial emulsions are sensitized with a sulphide of silver or silver gold.

Here is the link to the paper for review:

https://www.imaging.org/site/PDFS/Papers/1999/PICS-0-42/1072.pdf

Stephen Benskin · Feb 19, 2023

ISO 6 changed the hold time from greater than one hour and less than two to not less than 5 days and not more than 10 days after exposure for general purpose films and not less than four hours and not more than 7 days for professional films. Anybody remember seeing anything about hold time with Zone System testing?

ic-racer, I love how the author in the paper for the IS&T conference used Delta speeds.

I did a latent image test and then repeated it to confirm. These are the resulting times and speeds with 35mm TMX processed in seasoned T-Max R/S with a Refrema dip & dunk. Exposed using a calibrated EG&G Sensitometer and read on a MacBeth densitometer. For this type of test, I believe, the equipment needs to be more precise.

Helge · Feb 19, 2023

Lots of interesting thoughts and ideas here guys. Thanks.
Please keep going if you see fit.
Half a stop is nothing to sneeze at. And fast development can be used accumulatively with other methods to gain extra speed if need be.

ic-racer said:
I don't have a curve, per say, but you could make one from this data:

The table shows, for example, a mid value (0.5 h Point) can lose one-half stop (0.15 log E) if held in an 80% humid 20% oxygen/70% nitrogen environment for 72 hours.

Likewise, film held in a vacuum for 72 hours can gain 1/6 stop (0.05 Log E).

View attachment 329975
IS&T’s 1999 PICS Conference
The Effect of Environment on Latent Image Formation and Stability
Sean W. P. O’Toole Eastman Kodak Company Rochester New Y ork

Why would film get faster by simply being stored in a vacuum after exposure‽
First I ever heard of this effect.

ic-racer · Feb 19, 2023

Helge said:
Lots of interesting thoughts and ideas here guys. Thanks.
Please keep going if you see fit.
Half a stop is nothing to sneeze at. And fast development can be used accumulatively with other methods to gain extra speed if need be.

Why would film get faster by simply being stored in a vacuum after exposure‽
First I ever heard of this effect.

One conclusion is that the vacuum removes oxygen from the emulsion. With less oxygen, less degradation of latent image and greater speed.

Stephen Benskin · Feb 19, 2023

ic-racer said:
One conclusion is that the vacuum removes oxygen from the emulsion. With less oxygen, less degradation of latent image and greater speed.

I wonder if it's connect to some of the hypersensitization techniques that astronomers once utilized.

Doremus Scudder · Feb 19, 2023

aparat said:
It is labeled in such a way that it can be misleading. As I mentioned, the curve is theoretical and only shows the "gross" effect for comparison's sake. Here's the quote that accompanies the plot:

"Since the latent image consists of a cluster of silver atoms, the loss of only a few atoms may render a silver halide grain undevelopable, An atom of silver can combine with an atom of bromine, for example, to form a molecule of silver bromide - reversing the effect of exposure. After long periods of time the image, when developed, may have less contrast and be less distinct due to the spontaneous development of a large number of unexposed silver halide grains. As this type of development fog increases, a point is reached where the developed latent image can no longer be detected. A small amount of development fog, however, can produce the effect of increasing the speed of photographic emulsion and the density of the developed latent image in the same manner as latensification."

The highlighted portion in the text you posted above describes age fogging due to "spontaneous development of a large number of unexposed silver halide grains." That really has nothing to do with latent image degradation, which is described in the first sentence. The graph itself shows age fog degradation, i.e., a boosting of the shadow values due to the fogging. Latent image degradation would affect the highlights proportionally more. This is not shown on the graph.

Best,

Doremus

Doremus Scudder · Feb 19, 2023

Stephen Benskin said:
ISO 6 changed the hold time from greater than one hour and less than two to not less than 5 days and not more than 10 days after exposure for general purpose films and not less than four hours and not more than 7 days for professional films. Anybody remember seeing anything about hold time with Zone System testing?

ic-racer, I love how the author in the paper for the IS&T conference used Delta speeds.

I did a latent image test and then repeated it to confirm. These are the resulting times and speeds with 35mm TMX processed in seasoned T-Max R/S with a Refrema dip & dunk. Exposed using a calibrated EG&G Sensitometer and read on a MacBeth densitometer. For this type of test, I believe, the equipment needs to be more precise.

View attachment 330028

It looks like the difference in film speeds at both ends amounts to about 2/3 stop. Less if one considers that film is rarely (if ever) processed within 30 seconds of exposure in practice. The difference between one hour and 31 days is about 1/3 stop. Not a lot of incentive to rush to develop the film, especially if one gives an extra 1/3 stop exposure when making the negative.

That said, I believe there are other films that show more latent image degradation than the one you tested, e.g., Ilford Pan-F.

Doremus

Bill Burk · Feb 19, 2023

I’m surprised the speed of TMAX 100 found by Stephen was “about 64” under what I would think of as normal circumstances.

Curve of the fading of the latent image in modern emulsions? And the possible speed gain by developing really quickly?

Privacy & Transparency

Privacy & Transparency