Why is home processing for color not nearly as wide spread?

[Mr Bill]

Well, I was once in a minilab chatting with the owner and watching a girl operating their printing machine. That was back in the late 80's and the machine wasn't too automated so the girl had to manually expose each frame on the film. She made adjustments (filtration and exposure) when she felt necessary. I thought that was amazing and the owner confirmed to me that she could visually look at the projected negative image and make filtration and exposure changes almost right on. So I think with training it is possible to build up the skill.

Hi, I've been involved, to a pretty serious degree, with using the lab technologies of the day, since well before one-hour labs came about. So I'm comfortable saying that printing on a typical mini lab of that Era is not really equivalent to being able to make a direct visual judgment of what color printing filtration is needed. A little off-topic meandering is coming up...

In the case of the mini-lab, almost all the earlier machines used something like a 3-color "averaging" sensor behind the negative to more or less measure the average color coming through the negative. The machine would loosely use an idea known as "integrate to gray," meaning that it sort of assumes that the scene is supposed to average out to a neutral color, more or less. But often this is not the case, and the machine screws up, so to speak. In the business they called this "subject failure," but obviously it was really a failure of the machine to deal with a non-average scene. For example, if you had photographed a person dressed all in red, the machine wants to cancel out enough red to make the scene neutral. So it essentially would try to add cyan to the scene, making the final print have cyan skin tones (a really bad way to go).

So it was the job of the 1980s mini-lab operator, using "integrate to gray" technology, to view each negative and decide whether or not the automated system was gonna work ok, or not. If not, they would try to estimate how far off the machine would likely be, then override it's automatic response with a rack of buttons.

It's not unlike what a photographer would do with a reflective meter in non-average scenes. If you meter a snow covered landscape, for example, you think, hmmm... the meter expects a mid gray scene; this is mostly white, so I should increase the exposure by maybe 2 or 3 stops, depending.

But back to the mini-lab operator... they are looking at the negative and making a judgment, not of what filtration is actually needed, but rather what will be the error of the automated system. And what buttons do they push to override it. That is what they were doing. They could make these judgments by observing some of the things that koraks has mentioned, but again, they are not correcting strictly for the actual difference, but rather for the errors that the machine would make. Fwiw there were a lot of people that got really good at this, I'd guess maybe one in ten, or so, maybe one in five, depending on what one calls "really good." (Although even the best mini-labs fell significantly behind manually hand-corrected pro labs.)

There's more to it than that, though, again going on behind the scene. I don't wanna explain too much here, but from the early days of automated printing there was a thing called "slope control," which gave the machine the ability to automatically adjust for reciprocity failure. It had to be set up by using a set of "printer control negatives," an exposure series on each specific film type that one might encounter. Then, in normal operation, the operator selects the proper "channel" for the film they are printing, and color errors for over and under exposed negs are automatically corrected, more or less.

Somewhere along the line Agfa came out with the first "scanning" mini-lab machine, the first MSC model, and machines could become "smart," and get away from the "integrate to gray" method. Pretty soon retro fit systems became available for older machines, and skilled printer operators were not so crucial. And competitive pressure soon meant that mini-labs wouldn't be able to pay higher wages for highly-skilled operators, etc., who eventually went elsewhere, and so on.

Anyway, the bottom line is that skill in operating one of those mini-labs does not equate to the ability to directly judge printing packs needed for a specific negative.

 

[MattKing]

In the case of the mini-lab, almost all the earlier machines used something like a 3-color "averaging" sensor behind the negative to more or less measure the average color coming through the negative. The machine would loosely use an idea known as "integrate to gray," meaning that it sort of assumes that the scene is supposed to average out to a neutral color, more or less. But often this is not the case, and the machine screws up, so to speak. In the business they called this "subject failure," but obviously it was really a failure of the machine to deal with a non-average scene. For example, if you had photographed a person dressed all in red, the machine wants to cancel out enough red to make the scene neutral. So it essentially would try to add cyan to the scene, making the final print have cyan skin tones (a really bad way to go).

When I ran a machine like this, we were doing colour and exposure corrected proofs and machine enlargements for professional (mostly wedding and portrait) photographers.
The Durst mini-printer machine I worked with had a "test print" mode - it would print an approximately 1 inch wide angled strip across a 4" x 4" section of the 4"x4", 4"x5" or, rarely, 4"x 6" proof. The exposure and colour for that was set by the averaging sensor.
I would do that for several rolls, develop the long section of roll paper in the large, automated roller transport paper processor (kept within spec by a minimum of daily control strips) and then evaluate each strip for exposure and colour balance. I would mark the necessary red, blue or green (IIRC) corrections on the face of the strips with a marker pen.
Then I would take out the test print mask from the machine and replace it with the print mask. The negatives went back through the procedure, and for each one I would read my marked corrections on the test and set the necessary corrections on the machine, than expose the print.
The Durst Mini-printer was the size of a small desk and had controls and negative holders on the front side and a light tight paper transport compartment on the back side. The mini-printer was installed in our work area with the front out in the general, fully illuminated part of our workplace, while the paper transport area of the mini-printer was through the wall and inside the darkroom. The roller transport processor we used was set up in reverse, in that the paper feed area was in the darkroom, and the output area for dried prints was through the wall and out into the light. The darkroom also housed the large (8"x10" IIRC) colour enlarger - probably a Durst.
The mini-printer's light tight paper transport area was big enough to hold a large roll (300 feet?) of paper up to 11 inch wide (IIRC).
After a little while, I was able to make most prints with just the test and one set of marked corrections - reprints were infrequent.

 

[Bikerider]

Yes sure but if you're using it intermittently you have to PH check, replenish etc. so it's really not trivial. B&W is just one-shot and lasts forever unmixed.

Like Koreks I have no issue with both C41 developer or RA4. C41 I have always used in a use once and throw away basis so every drop of developer is fresh. RA4 has few problems so long as it is replenished on a strictly regular basis (100cc per every 80 sq ins of paper). I usually only do a full change in my Nova when it is getting very dirty with tar and that has been as long as a year or a little longer.

Using an open dish for the developer risks the developer going off sooner than need be, because in an open dish it will absorb oxygen quickly and then start to deteriorate. People have mentioned that it is not cost effective (colour processing), Fair enough if you are doing it commercially but for me it is a hobby and I don't usually count the cost.

I know folk who list drinking beer as a hobby and they don't count the cost either - would you!

 

[unityofsaints]

I don't have an issue with RA-4 development either but people look at stuff in theory before they do it and there's no denying that, in theory, RA-4 printing is more complicated than B&W - not to mention the scare stories out there about temperature control, smells and unfixable colour balance.

 

[Mr Bill]

When I ran a machine like this, we were doing colour and exposure corrected proofs and machine enlargements for professional (mostly wedding and portrait) photographers

Yeah, but the sort of thing I meant by mini-lab was one of the one hour labs, where yours was more of a pro-lab sort of thing, doing manual corrections. So everything that goes out the door, you KNOW that it has high quality color, at least within a tolerance.

The standalone one-hour labs had to keep pumping amateur work through at a fast enough pace to run a successful business, so for the most part had to use the automation to make a more or less acceptable first-run print. Back in the day the operator would typically check the final prints, then if several were too far off, whatever "too far" means, they could reprint before the customer got back. (The "button settings" used were typically printed on the back, so this was a guide as to where to go with further corrections.)

When scanners came on the scene, and later the high res ones that could directly make digital prints, they could figure out most of the "subject failure" issues, so the really bad prints didn't need to happen anymore. The "best" prints didn't really get any better, but the real "dogs" no longer happened. So there was mostly ne real need to inspect all of the prints before releasing them. Plus, these systems could figure out where each negative started and ended, so with auto negative advance, there was no need for an operator to be sitting at the printer full time. So eventually it got to where almost no qualifications were needed for an operator.

 

[MattKing]

IIRC, the Durst also back printed the corrections set on the final prints.
I related all that because it may indicate a workflow possibility for those who have a colour enlarging meter available to them, and are prepared to calibrate their process using it.
Stick a diffusing integrator on the lens, determine the color head settings by nulling the meter, make a test, and then go from there.

 

[mtjade2007]

I don't. Sorry.
Training a human observer to accurately judge the need for a +/-10CC adjustment (still a fairly coarse adjustment, mind you!) based on just a projected negative - not a chance.

I myself was very poor in tweaking the filtration. I ended up giving up wet printing long time ago despite I had a decent set of color darkroom equipment. The owner of the minilab I mentioned was a distant friend to me. We played golf together a few times. So I was in his lab digging information trying to find out how his operators managed to print at least between 1200 - 1800 photos a day without getting a stroke due to elevated blood pressure setting filtration for each photo. I was told that the lab had to have minimum of 50 rolls (35mm) of business a day to barely make a profit. Obviously they could not have more than a few reprints after a roll of film was printed. They basically printed the whole roll then inspected each print and determined which print needed a reprint.The printer had mechanism to quickly make reprint on any particular frame. The film was then cut and sleeved after that.

This was in the mid and late 80's. I guess the printer had a mechanism to set the filtration fairly close to what it needed to be. The operator probably only checked if the skin tone was right and the shadow was neutral, which was easier to be trained. They did not really need to adjust the filtration for each frame. Rather, it was probably set for the entire roll and had to reprint a few photos only. So I agree with you, Koraks, The mini lab operator is a human still and there is no way they can set the filtration right on in one shot.

 

[RPC]

I don't have an issue with RA-4 development either but people look at stuff in theory before they do it and there's no denying that, in theory, RA-4 printing is more complicated than B&W - not to mention the scare stories out there about temperature control, smells and unfixable colour balance.

Some people have trouble with learning to color balance prints, but the actual the RA-4 process can be simpler than b&w. With b&w printing you need three solutions, developer, stop bath and fixer. RA-4 in trays requires no stop bath or pre-wet (as drums do), so all you need is two solutions, developer and bleach-fix. With Kodak RA-RT Developer/Replenisher you can develop at room temperatures (68F-75F) for two minutes with excellent results, so no strict temperature control is needed as with C-41. I have never had a problem with unfixable color balance, but it can happen if the film or paper was expired or was not processed properly, all of which can be avoided with diligence. Any issues with smell can be controlled with adequate ventilation.

Incidentally, the Kodak developer works very well for tray use as it has excellent oxidation protection. You can get as many prints before it has reached its capacity in an open tray per a given volume as you can if used one-shot in a drum.

 

[unityofsaints]

To each their own but I'd much rather run B&W without stop than RA-4 without stop! In B&W you just exhaust your fix quicker, with RA-4 you can ruin your print (depending on freshness of chemistry and a bunch of other factors of course).

 

[Mr Bill]

So I was in his lab digging information trying to find out how his operators managed to print at least between 1200 - 1800 photos a day without getting a stroke due to elevated blood pressure setting filtration for each photo.

Well heck, I could've told you how they can do that. In fact I actually did just explain how the systems work. On the internet it's hard to know who to trust. I once read, on the early internet, a guy saying that if you already know how to do something it's easy to find the right answers on the internet. But if you don't already know how, it can be really difficult to find the right answer.

Fwiw I spent years as the QC manager in the main processing lab of a large studio chain. We did QC primarily on the chemical mixes and process control, not on the "production" printing. But I did have a full-time product inspector, doing semi-random inspection on work ready to ship. Now, this inspector only saw a couple percent of the total production, but this was statistical "quality" reporting for management, not for quality screening of everything.

All told, we (meaning the production departments) were printing, inspecting (for color or physical defects), dust "spotting" and shipping roughly 500 to 1,000 times more paper quantity (square footage) than your friend's mini-lab. And we were hand-correcting the color within a tolerance. Now, we had one big advantage over that mini-lab - the studio chain was company owned so all the studios were set up to certain specs, using long roll cameras, etc., so we could count on the great majority of the negatives within a roll being fairly consistent. So you color balance for a roll, then do remakes based on inspection of all production. In fact, any person on the line - full-roll inspectors, dust "spotters" (they retouch white specs due to dust), or the packagers - could kick-back a print unit for remake. (We did nominal 8x10" portrait units, so the larger units are more sensitive to dust spots. Mini-lab prints, on the other hand, use lesser enlargement, so much less issue with dust spots.)

Fwiw, the company, at one time, also owned the largest chain of one-hour labs in the US. Although it was a different division than where I worked, this is largely where my knowledge of one-hour labs comes from. We had a good working relationship between the groups, at least in the tech areas, so we could share expertise where useful. Fwiw, our main lab still printed more paper than the entire mini-lab chain. Although they processed a much greater amount of film. (They made small prints of every negative; we printed portrait "packages" from select negatives, only.)

Having said this, it being the internet and all, I could be making this all up. Maybe, maybe not; it's hard for the casual hobbyist to really know for sure. So who you gonna trust? I learned, long ago, that if someone knows just a little more than you do about a subject, it's possible for them to carry on a deception. Maybe deception is not the right word; there are many well-meaning people, trying to be helpful, who believe things that they "know" are true. Now personally, I believe that everyone's personal experience is valid - what they experience is true. But problems can come if they try to interpret what they experienced. But I digress; I probably meandered on too much.

Bottom line is that the operators of early one-hour labs were not actually calling out color corrections based on the appearance of the negative. They were calling out the situations where the automated printing system was likely to be fooled, then estimating the direction and extent of the likely error. And... some people learn to be really good at this; I mean really, really good. Your friend's printer may have been one of these people. On the other hand, maybe your friend's own "quality judgment" was poor, so the printer "seems" to be exceptional. My experience, or interpretation of it, is that you often need to see a wide variety of "performances" in order to judge the ones that are really exceptional; a lot of it is relative.

Back on the topic of, can a printer actually tell, just by looking at a negative, what the printing pack should be? I have never actually observed this, so I can't say for sure. The work we did was first run on a Kodak VCNA or PVAC, very expensive video negative analyzers, to get starting filter packs. I've worked with dozens of professional color correctors over the years. But all of them were working from a test print - they didn't look at actual negatives. But a lot of them are really good - you can't hardly believe it until you see them work. You might read on the internet, how to color correct. Typically it will say, "make only one change at a time." Or "use your 'print viewing filters' like so," and that sort of thing. Professional color correctors don't work that way. They routinely call out things like two colors and a density correction at the same time. They'll generally be really close, and the best will just about nail it, certainly within 2cc, every time (depends how far off they're coming from). Now, the pro correctors are generally doing this on a slope-corrected machine, so they don't have to deal reciprocity-related shifts, however significant they may be.

Now, if we could set up a test, under controlled conditions, to view some random color negative and guess at a filter pack, I would absolutely NOT bet against some of these color correctors. Of course they would have to get the opportunity to practice for some considerable time, and there should be a "standard" negative for reference, and some predetermined tolerance to hit. So such a thing will probably never actually happen. But like I keep saying, some of them are so good you can't hardly believe it. And it's possible, although unlikely, that the mini-lab operator in question is one of these people.

 

[sillo]

Some people have trouble with learning to color balance prints, but the actual the RA-4 process can be simpler than b&w. With b&w printing you need three solutions, developer, stop bath and fixer. RA-4 in trays requires no stop bath or pre-wet (as drums do), so all you need is two solutions, developer and bleach-fix. With Kodak RA-RT Developer/Replenisher you can develop at room temperatures (68F-75F) for two minutes with excellent results, so no strict temperature control is needed as with C-41. I have never had a problem with unfixable color balance, but it can happen if the film or paper was expired or was not processed properly, all of which can be avoided with diligence. Any issues with smell can be controlled with adequate ventilation.

Incidentally, the Kodak developer works very well for tray use as it has excellent oxidation protection. You can get as many prints before it has reached its capacity in an open tray per a given volume as you can if used one-shot in a drum.

This is my experience too. You can even add a safelight too if you want. It's not the most useful, but when your eyes adjust it can help get the hang of things. With the help of some old Kodak print view filters even the color balancing isn't terribly hard, plus the paper is cheap so wasting a few test strips really isn't a big deal.

 

[faberryman]

It has been so long ago I can't remember how I arrived at the right color balance on my Cibachrome prints. I had an Omega B22XL enlarger so it was obviously just sticking filters in the filter drawer. I know I didn't have a color analyzer. I guess I just guessed, and hundreds of dollars in chemicals and paper later I figured it out. There is probably an app for color printing now. Just stick your phone on the easel and you are good to go. Of course you would still have to hunt down a set of filters on eBay.

 

[Bikerider]

It has been so long ago I can't remember how I arrived at the right color balance on my Cibachrome prints. I had an Omega B22XL enlarger so it was obviously just sticking filters in the filter drawer. I know I didn't have a color analyzer. I guess I just guessed, and hundreds of dollars in chemicals and paper later I figured it out. There is probably an app for color printing now. Just stick your phone on the easel and you are good to go. Of course you would still have to hunt down a set of filters on eBay.

Even an app would probably find it difficult unless you were able to input certain information regarding the initial developing of the film which could be wrong anyway. Cibachrome has not been available for many years now but an ordinary colour enlarger would perfectly well work if it did

 

[Bikerider]

IIRC, the Durst also back printed the corrections set on the final prints.
I related all that because it may indicate a workflow possibility for those who have a colour enlarging meter available to them, and are prepared to calibrate their process using it.
e.

In fact that would only apply for accuracy if the paper, developer and colour balance of the light source were identical to the one you were using. Get any of them wrong and it is like falling off a tightrope.

 

[Mr Bill]

I myself was very poor in tweaking the filtration. I ended up giving up wet printing long time ago despite I had a decent set of color darkroom equipment.

Hi, fwiw you may have had a color vision deficiency; hard to be sure without taking an actual test. Where I worked, my department actually gave such tests to people being considered for a color-correcting job. We found that the most useful test was the one linked below: (Farnsworth-Munsell 100 hue test)

https://munsell.com/faqs/what-does-score-farnsworth-munsell-100-hue-test-mean/

Essentially there is a set of color samples that the tested tries to put in order. They use a moderate amount of color saturation in a wide range of hues. Each set has the first and last "cap" fixed in place, then one tries to put every other one in sequence. If a person DOES have a significant deficiency they will see a handful of the colors as being identical, so will not be able to get the correct order (aside from pure luck, which is really unlikely).

We had test records from something like 60 or 80 people (with two tests per person). And there were a wide range of results, with probably only about 5 or 10 perfect scores. The test typically took about 15 or 20 minutes, but no hard time limits. Now I, personally COULD get a perfect score IF I took a lot of time and triple-checked the sequences, but this is NOT the proper way to do the test. In a normal situation I would probably have 2 or 3 wrong pairs, which certainly would be good enough for color correcting. But there were SOME people who could just go bam!, bam!, bam!, be done in less than 5 minutes, and have a perfect score with no doubt in their mind about it. So clearly these people had extraordinarily good color discrimination, much better than is reflected in a perfect score.

A second thing that happens is, if there is not a high-quality light, with full spectrum, such as daylight, you would also not be able to get a perfect score.

Now, it is not necessary to have a perfect score, or even a high score, to do a pretty decent job of color correcting. But if there is a significant deficiency, such as would show up on a color blindness test, you'd likely have a lot of problems. Not always, because you might have a scene with colors you are OK with, and you might make your judgment based on those. But in some cases you'd be powerless to make accurate corrections. Because some of the colors will look the same to you. You would only know they're off if someone else told you.

If someone wanted to make their own home-brew test, I'd suggest printing a narrow-range ringaround of some pastel-like colors. Perhaps the ring is in 2cc color units, ie, +2red, +2 green, +2 blue, then -2 red, etc. Then see if you can consistently put them in the correct order. If not, then you might have a color vision deficiency. But... it's possible that your printing system was not repeatable enough to get consistent differences. So, you could have someone else, hopefully with good color vision, put them in order. Fwiw, strong color deficiencies are almost exclusively a male trait, statistically in about 5 to 10% of males. Whereas virtually no females have such an issue.

Now, this thing about whether or not you have really fine color discrimination is a separate issue from a color blindness, so doesn't seem sex related. So perhaps a person can train themselves to become better with this (although not with actual color blindness); I dunno know for sure.

Going a bit further, I would say that a person with significant color deficiencies doesn't necessarily have to give up on color printing. Lacking someone else to call the shots for them, there are a variety of instruments that can measure a printed result. But you DO have to know what the readings mean, and what the expected color is. Ideal would be a handheld spectrophotometer that can give readings in CIELAB; this would never be fooled. But you have to learn what the values mean. I think it's likely that a digital camera might work decently too, but there might be certain cases where the camera could be off - this is because digital cameras typically "see" differently than the human eye, on a spectral basis. But my point is, a color blindness doesn't necessarily preclude one from color printing.

 

[MattKing]

In fact that would only apply for accuracy if the paper, developer and colour balance of the light source were identical to the one you were using. Get any of them wrong and it is like falling off a tightrope.

The commercial equipment, supplies and techniques we employed ensured that consistency.
The back printing was an accurate indicator within our systems, and would have been useful if I had had to try to emulate results in a different, analyzer equipped darkroom.

 

[koraks]

If someone wanted to make their own home-brew test

I like your idea - but for those who are somewhat lazy (including myself), there's the lightweight online test here: https://www.xrite.com/hue-test
By far not as extensive as the torture test you mentioned, but it's the same principle.

 

[RPC]

To each their own but I'd much rather run B&W without stop than RA-4 without stop! In B&W you just exhaust your fix quicker, with RA-4 you can ruin your print (depending on freshness of chemistry and a bunch of other factors of course).

In years of printing I have never had a print ruined. Re-adjusting the pH of the blix occasionally can make it last a long time.

 

[Mr Bill]

I like your idea - but for those who are somewhat lazy (including myself), there's the lightweight online test here: https://www.xrite.com/hue-test
By far not as extensive as the torture test you mentioned, but it's the same principle.

Hi, I like to think I'm as lazy as the best of em, but certain curiosities can drive one to, I dunno, almost addictive behavior, not unlike video games, I guess. Or anything to avoid real work.

Fwiw I have played with that online test a number of times over the years, and while it's sort of fun it can be kinda misleading. I think the first time I tried it was with a fairly high quality monitor, not profiled though. Moderate score, not too great. A handful of times on a laptop; visually, with images, it looks like it's a pretty decent monitor, but I got miserably poor scores. Visually it looked like I would have a fairly decent score, but the actual score was not even close. Finally, on a fairly recent phone, not an expensive one, either, a near perfect score.

So I think the online test is potentially detrimental to people. One might conclude they have poor color vision, maybe even avoid color printing, when the whole issue is perhaps due to their monitor. I think the way to make some conclusion about a poor score would be to have other people take the test on the same monitor. If they consistently outscore you by a lot, then maybe you have a color vision deficiency. Or maybe not; it's hard to really be sure.

Fwiw the real test is really not bothersome at all. The testee used to get a break from work for a while. (I say "used to" cuz the company went belly up some years back; bout the same time as Kodak.) They can relax a bit while they arrange some pretty colors. And the person giving the test can read a magazine for a while - the only reason they are there is to make sure nobody is looking at the numbers on the underside of the colors caps. Well, and they have to score the test afterwards.

 

[Bikerider]

The commercial equipment, supplies and techniques we employed ensured that consistency.
The back printing was an accurate indicator within our systems, and would have been useful if I had had to try to emulate results in a different, analyzer equipped darkroom.

I took you to mean if a home printer was to use these filtration figures they would get identical results. Take say the 55 M and 60Y printed on the back of the print on Fuji paper and tried the same when using Kodak paper which is what I was basing my comments on.

 

[faberryman]

So I think the online test is potentially detrimental to people. One might conclude they have poor color vision, maybe even avoid color printing, when the whole issue is perhaps due to their monitor.

Avoid analog color printing or digital color printing, or both? Might photographers who don't do well on the test avoid scanning color film, or working on color scans or digital color images in Photoshop too? Or give up photography altogether perhaps? I took the test a while back, probably on a laptop whose screen was not calibrated. I think I did okay but not perfect. The thought did not cross my mind to give up any aspect of photography. In fact, until you mentioned the test, I had forgotten all about it. I think the worst thing that could happen if you don't do well on the test is that you might end up buying some XRite calibration paraphernalia. Hmm. Maybe that's the whole idea being as the test is on the XRite website?

 

[MattKing]

I took you to mean if a home printer was to use these filtration figures they would get identical results. Take say the 55 M and 60Y printed on the back of the print on Fuji paper and tried the same when using Kodak paper which is what I was basing my comments on.

Actually, the numbers on the back from that system weren't CC numbers, they were offset numbers, and related to the system used on that Durst (and perhaps other Durst) mini-printers.
What I am saying is that if I had a calibrated setup at home, that made use of an integrating and averaging analyzer, it would be a reasonably easy process to create a translation guide from those offset numbers to CC numbers and stops of exposure, after which the numbers would be useful.
Not perfect, but usefully indicative. The sort of thing that could lead to a first test that was close and, most importantly, where exposure was close.

 

[Bikerider]

Actually, the numbers on the back from that system weren't CC numbers, they were offset numbers, and related to the system used on that Durst (and perhaps other Durst) mini-printers.
What I am saying is that if I had a calibrated setup at home, that made use of an integrating and averaging analyzer, it would be a reasonably easy process to create a translation guide from those offset numbers to CC numbers and stops of exposure, after which the numbers would be useful.
Not perfect, but usefully indicative. The sort of thing that could lead to a first test that was close and, most importantly, where exposure was close.

You have totally lost me. You obviously use a different set of parameters to me. I was not saying if you had a set up at home it would have been my conclusion if I saw the prints, so it would have not been not your conclusion. In fact I have Commercial Prints made a long time ago (my wedding photographs from 1972) where the filtration was marked in C and M values, but there again it was for the C22 process and not C41. Let that be the end of it. I am getting bored!

 

[MattKing]

Speaking very generally, a careful darkroom printer who also has access to back printed prints from a good lab can get some useful information from the back printed correction data.
But it would take a lot of calibration before it would serve as something more than useful information.

 

[foc]

I hope this doesn't confuse matters, but this is what the back print can look like from a film scanned and printed on a Fuji Frontier. (the software can be configured to show different info).

A quick explanation is the machine made a correction of -1cyan and -1 yellow (which would be the equivalent to +1magenta but with a density difference) and +3 density. There was no manual colour or density correction.

This information is important if a remake of the print is required due to colour or density failure.