Things I don't understand in two ISO standard papers for determining the ISO speed from curves

AbsurdePhoton · May 13, 2024

Hi, for my first post/question, this is a question for the software I am writing.

My aim for this question is to calculate ISO speeds from density curves.

I carefully read these two publications, and could apply them with success :
* ISO 6-1993 - Photography-B&W pictorial still camera negative film process systems-Determination of ISO speed
* ISO 5800-1987 - Photography-Colour negative films for still photography-Determination of ISO speed

But now I have a problem understanding something in these two other publications on the same subject :
* ISO 2240-2003 - Photography-Colour reversal camera films-Determination of ISO speed
* ISO 7187-1995 - Photography-Materials for direct-positive colour-print cameras-Determination of ISO speed

These publications can easily be found as 'preview' versions, but I can't provide a link to them for legal reasons.
These last two documents are about color reversal films, and 'instant' color films.

In the process, the authors use only ONE density curve.
But... color films technical sheets show 3 curves (blue, green, and red), never one? How come?
Comparatively, in ISO 5800:1987, they explicitly use the 3 curves: you have to pick the one with the lowest D-min, then combine the result with the green curve as a reference for ISO rating.

So my question is: how can I interpret that ? Should I use a mean of the 3 curves, a sum, only one?

Thanks in advance.

Chan Tran · May 13, 2024

For color negative film because of the orange masking the 3 curves are separated by the masking so they use the lowest density curve. For transparency the 3 curves are pretty much on top of each others.

BrianShaw · May 13, 2024

Two comments regarding using standards.
1. When using standards in new work, the most recent version should be cited. In all cases, these standards have been confirmed as part of the routine 5-year review cycle. While 'confirmed" means reviewed, found to still be valid as wriiten and not updated, the most recent date should be used so as to not confuse the reader. (For instance, I looked up all four standards to check if they had been updated or not; minor inconvenience but...
2. While the leading zero in " ISO 07187-1995 - Photography-Materials for direct-positive colour-print cameras-Determination of ISO speed" is arithmetically insignifiant, it inhibits a google search for the document. Do not add the leading zero as the number of the documents is "ISO 7187-####".

I would hope that a reading of the full standard would reveal how the curves should be arithmetically combined. But they are expensive to buy so I understand...

Andrew O'Neill · May 13, 2024

Sorry folks, I accidentally deleted this thread. Thanks Brian, for the heads up! Now where'd I leave that coffee cup of mine....

AbsurdePhoton · May 13, 2024

Sabotage! It was my first question here, I registered 2 hours ago!!! I even wrote an email to the administrators to know what I did wrong. Please ignore it, LOL.

So let's come back to the question.

Thanks for your comments, both, that was fast! (I didn't talk about YOU Andrew )

I edited the question to delete the bad leading zeros: at first I copied the titles from the file names I had and corrected the first two ones but not the others, sorry. As for the dates, some were indeed confirmed in 2021 but not revised, so the references are searchable. Just a point of detail, I will do better next time, and I have an excuse: I'm far from from New York

@Chan Tran, I am aware that the three curves are mostly on top of each others, it is (almost) true for D-min, the toe and the straight line, but there is sometimes some divergence for the shoulder and D-max, e.g. the Polaroid 'spectra' series, the green curve is well apart, above the others.

@BrianShaw, before you edited your answer, you were talking about the curves, keeping the "most conservative one", do you mean keeping the curve with the lowest D-min / toe / shoulder / straight line?

This is something that is not specified in the documents (yes I have them, full version) --- both your comments make sense to me.

BrianShaw · May 13, 2024

AbsurdePhoton said:
@BrianShaw, you're talking about the "most conservative one", do you mean keeping the curve with the lowest D-min / toe / shoulder / straight line?

This is something that is not specified in the documents, but both your comments make sense to me.

I deleted that part of my reply (was in the process of deleting when...)

BrianShaw · May 13, 2024

AbsurdePhoton said:
This is something that is not specified in the documents (yes I have them, full version) --- both your comments make sense to me.

Refer to ISO-5800 clauses 5.6.3 and 6.2 of ISO-5800.

AbsurdePhoton · May 13, 2024

BrianShaw said:
Refer to ISO-5800 clauses 5.6.3 and 6.2 of ISO-5800

My problem is not with ISO 5800 or 6, but with 2240 and 7187. I have successfully applied ISO 5800 and 6.

Quote from §6.2, this is the standard procedure : process multiple films from "various batches of the product when selected, stored, and tested as specified above. The ISO speed of a product with proper rounding is then determined from the average value of log10Hm, by use of the table".
About the "average value", they're not talking about the blue green and red curves, but the average values from different batches.

But, thinking about that: the differences between the 3 curves are not huge, so an average of them would make sense too. The big differences I saw were mostly for the shoulder of reversal films, which doesn't have an influence at the other end of the curve for the ISO calculation.

BrianShaw · May 13, 2024

I'm confused by your actual question, it seems. Are you trying to figure out why ISO-5800 discriminates on color channel but ISO-2240 and ISO-7187 don't?

Also, when quoting a section of a stanard, please include which standard you are quoting. You appear to be quoting the same section of a different standard than I quoted, probably ISO-7187; the section you quoted is on a different topic - accuracy.

AbsurdePhoton · May 13, 2024

BrianShaw said:
I'm confused by your actual question, it seems. Are you trying to figure out why ISO-5800 discrimiates on color channel but ISO-2240 and ISO-7187 don't?

AbsurdePhoton said:
But now I have a problem understanding something in these two other publications on the same subject :
* ISO 2240-2003 - Photography-Colour reversal camera films-Determination of ISO speed
* ISO 7187-1995 - Photography-Materials for direct-positive colour-print cameras-Determination of ISO speed
...
These last two documents are about color reversal films, and 'instant' color films. In the process, the authors use only ONE density curve.

Exactly, I am trying to figure out why ISO-5800 discriminates on color channel but ISO-2240 and ISO-7187 don't. and so, how to do the calculation, if I don't know which curve to use.

Andrew O'Neill said:
Now where'd I leave that coffee cup of mine....

Brian, maybe you should have a cup of coffee too? No worries, my native language is not English and my manner of asking the question may be confusing.

BrianShaw · May 13, 2024

AbsurdePhoton said:
Exactly, I am trying to figure out why ISO-5800 discriminates on color channel but ISO-2240 and ISO-7187 don't. and so, how to do the calculation, if I don't know which curve to use.

Brian, maybe you should have a cup of coffee too? No worries, my native language is not English and my manner of asking the question may be confusing.

Had one cup of coffee; two would make matters worse.

You are doing fine in English. I'm a stickler for accurate and precise wording to ensure that the communication is efficient. My last boss would laugh in my face and, jokingly, mock me because no matter what he said I'd likely ask for more specific clarification. We actually worked well together because I knew I couldn't read his mind so I didn't even try.

Having spent my entire career in various aspects of engineering standards, including generating basic data, using standards, authoring standards, and managing standards development, I can assure you that two things are very challenging: crystal clarity of the requirements and alignment of content between standards. Personally, as a standards user I take each for what they are and trust the standards development process.

I'd guess that there simply was not enough value in discriminating between the three color channels and all but one authorship team aknowledges that. One way to figure it out is to do historical analysis of each standard. Standards tend to have a certain stability that represents the thought process of the authorship team. The thought process, though, often is not documented inthe standard so it's quite a mystery, especially as authorship teams evolve over the various editions. Plus, there is not always a unanimous opinion since it only takes a 75% approval vote. Often there are 'sour grapes' that pesist over specific details, sometimes for a long, long time. At the "national level" of standards development, we were always encouraged to "explain the details" in an associated guide/handbook. I don't know if ISO-level guides/handbooks exist on these topics, though; I doubt it.

AbsurdePhoton · May 13, 2024

And so my question will remain without an exact answer, but you helped me think about it.

If nobody here thinks otherwise, I think I will take the average of the three curves, anyhow the location of Hm is far from the divergent shoulders, so that should do the trick.

But when you think about it, standards should not leave way to interpretation like these two.

BrianShaw · May 13, 2024

AbsurdePhoton said:
And so my question will remain without an exact answer, but you helped me think about it.

If nobody here thinks otherwise, I think I will take the average of the three curves, anyhow the location of Hm is far from the divergent shoulders, so that should do the trick.

But when you think about it, standards should not leave way to interpretation like these two.

But they don't leave anything to interpretation. Measurement on separate color channels is not a part those standards.

ISO-7187 Section 5.5, Densitometry specifies how to measure an instant print. Perhaps there is some clue to be found in the two referenced standards, which are compliance standards. Same with ISO-2240 Section 5.5, Densitometry, for color positive film. I did not track down those referenced standards to see if htey mention meaurement of separate color channels but that would be a useful exercise.

Note that ISO-5800 does not include those second tier standards. It is possible that the applicable clauses were incorporated into ISO-5800 to reduce the complexity of tiered standards. Tiering is generally problematic in terms of both use and standards maintenance.

BrianShaw · May 13, 2024

There's an answer, I'm sure. Perhaps one approach would be to ask why ISO-5800 is the only standard that measures three colors rather than asking why the other three don't.

The Technical Committee (TC) should be able to point you to the appropriate Working Group (WG) or Joint Working Group (JWG) for that standard.

ISO/TC 42 - Photography

Standardization primarily, but not exclusively in the field of still picture imaging - chemical and electronic - including, but not limited to: definitions for still imaging systems; methods for measuring, testing, rating, packaging, labelling, specifying and classifying the dimensions, physical...

www.iso.org

Alternatively (and I don't know how to do this so it's just an idea) is to trace backward. All ISO standards start as a national standard, like ASA.BS/DIN, and evolve from there in the international forum. If you could find out the history and which national stanrds was the origin then there might be meaninful clues as to both history and WG mindset. But htat is very hard work and might not be worth the effort.

AbsurdePhoton · May 13, 2024

Nah it's not worth the effort... because that's a lot of money too. I'll try to ask the committee instead.

In the meantime I'll just take the average of the three curves and see if the results are in accordance with official tech sheets like those from Fuji or Agfa.

Thanks for your help!

BrianShaw · May 13, 2024

But don't discount the notion that @Chan Tran had the explanation in post #2.

AbsurdePhoton · May 13, 2024

yes thanks again @Chan Tran

Mr Bill · May 13, 2024

Hi, with respect to the color reversal film speed standard, my understanding is that the densitometer being used should have what they call a "visual response." So there are no individual color density values to work with, only a single set which approximates the spectral sensitivity of the human eye.

I am getting this from IS&T's "Handbook of Photographic Science and Engineering," 1997 edition. (They have a brief explanation of each of the standard film speed methods.) I've not seen the current ISO standard, but would guess that the "visual response" densitometry is specified in there.

Ps, welcome to the forum.

AbsurdePhoton · May 13, 2024

Aaah, thanks, that is interesting, and would explain why there is only one curve. I have to find this book, now, looks interesting.

I also asked the ISO committee, by the way.

RalphLambrecht · May 13, 2024

AbsurdePhoton said:
Hi, for my first post/question, this is a question for the software I am writing.

My aim for this question is to calculate ISO speeds from density curves.

I carefully read these two publications, and could apply them with success :
* ISO 6-1993 - Photography-B&W pictorial still camera negative film process systems-Determination of ISO speed
* ISO 5800-1987 - Photography-Colour negative films for still photography-Determination of ISO speed

But now I have a problem understanding something in these two other publications on the same subject :
* ISO 2240-2003 - Photography-Colour reversal camera films-Determination of ISO speed
* ISO 7187-1995 - Photography-Materials for direct-positive colour-print cameras-Determination of ISO speed

These publications can easily be found as 'preview' versions, but I can't provide a link to them for legal reasons.
These last two documents are about color reversal films, and 'instant' color films.

In the process, the authors use only ONE density curve.
But... color films technical sheets show 3 curves (blue, green, and red), never one? How come?
Comparatively, in ISO 5800:1987, they explicitly use the 3 curves: you have to pick the one with the lowest D-min, then combine the result with the green curve as a reference for ISO rating.

So my question is: how can I interpret that ? Should I use a mean of the 3 curves, a sum, only one?

Thanks in advance.

Stephen Benskin is your man to ask.

Mr Bill · May 14, 2024

AbsurdePhoton said:
Aaah, thanks, that is interesting, and would explain why there is only one curve. I have to find this book, now, looks interesting.

I also asked the ISO committee, by the way.

Hi, I just did look at a preview copy of ISO-2240-2003, for color reversal films. In the section called "Densitometry," it says, "ISO standard visual diffuse transmission density of the processed images shall be measured..." In other words this confirms the use of a so-called "visual response," a broad spectral response approximately similar to what humans see.

The IS&T Handbook suggests that this is due, at least as of 1997, to film evaluation normally being done by people viewing either on a light box or projection screen. So it seems reasonable to use a densitometer with similar spectral response.

Now, in my view this is fundamentally different from the color negative systems in that a color reversal film is a final product by itself. For example, if one photographs a neutral gray-scale image the expectation is that the reversal film will make a neutral gray reproduction as the end result. Whereas a color NEGATIVE is an intermediate step; it doesn't need to be neutral on its own. As mentioned earlier in this thread, the use of an orange mask in color negative pretty much guarantees that it is not visually neutral. Thus the use of individual color density values.

AbsurdePhoton · May 14, 2024

Many thanks (again) for that!
So I have to find a way to combine the 3 curves, test with tech sheets data from several brands, see if I find the same results, try again if not.

Mr Bill · May 14, 2024

AbsurdePhoton said:
Many thanks (again) for that!
So I have to find a way to combine the 3 curves, test with tech sheets data from several brands, see if I find the same results, try again if not.

You're quite welcome.

I'm not real clear about what you're trying to do, though. I've used probably several dozen different color densitometers over the years. As I recall all of them also had the capability of using the visual response when reading density. So to me this is the obvious way to get such readings. (I sorta presumed that either you or a customer would be reading their own sensitometric test strips.)

If you're trying to use existing tricolor densities to estimate the equivalent visual density this is not really a proper way to do it. The combined dyes that look neutral to a human would actually make a wiggly line if you could graph the dye transmission vs wavelength of light. Color densitometers look at a set of three fairly narrow spectral zones so they are not able see what's outside this. But a "visual response" can see spectral areas that the color densitometer is blind to.

That said, most likely the film dyes are well behaved and likely have spectral peaks close to what the color densitometer sees (Status A response is normally used for color reversal film). So I'd guess that you'd probably be successful by either averaging the tricolor densities OR perhaps just use the green plot (magenta dye) only.

Best of luck!

AbsurdePhoton · May 14, 2024

I'm just trying to get the most of density curves, and the more I investigate, the more I learn too. The question I asked is not really essential to what I'm doing, it is more about curiosity and checking out what brands announce in their technical sheets vs reality - and of course beeig able to analyze home/self-plotted curves. There are also other parameters that can be deduced like gamma, density and exposure ranges, etc.

For those who could think I'm going too technical, the global aim is in fact artistic. To be able to get cool stuff, the technical side must be perfect and in appearance "get out of the way".

I have several other questions to ask, but I must admit this one proved to be much more interesting than I thought

(this said, I definitely must get the IS&T Handbook that I didn't know existed before yesterday, from what I searched and read it seems to be really amazing)

Mr Bill · May 14, 2024

Gotcha! I was primarily a photographer who took a "temporary" excursion into photo lab work to learn as much as I could about color film processing and printing, which seemed something of a black art in those days (prior to 1-hour labs, etc.). But I eventually found these things to be more interesting and challenging than pure shooting, which I never made it back to. The earlier version of the Handbook (1973?) was something of a bible to us. Originally published by the SPSE which later changed its name to IS&T. Pretty dry reading, but no other reference like it.

Things I don't understand in two ISO standard papers for determining the ISO speed from curves

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