Why are filters required for colour printing?

I've always just accepted the fact that when printing colour you have to dial in filtration as just that - an unquestioned fact and never thought any more about it. It was only last night when I was lying in bed that I started to wonder why we actually need to use filters. I mean we don't use filters to expose the negative in the first place (well ok I know we do to correct colour balance, etc under certain circumstances but you know what I mean.) so why do we need them when we print it.

Surely if the negative is correctly exposed and the paper is correctly exposed then shouldn't that be it? I can understand the need to perhaps tweak filtration say between differing brands as Kodak film and papers will have different characteristics to Fuji but why can't you just stick a Kodak negative in the enlarger, some Kodak paper on the baseboard, adjust exposure for the required density and that's it?

Wiggy;

I have said this before.

Enlargers vary in the hue of the light they output due to the optics and light source. Once you have the filtration for your enlarger, then any negative exposed in the last 50 years or so (except for Type CU Kodakcolor) will print within about a 10 R filter of that center point. I have over 1000 proof sheets to prove that point.

The major fluctuation in filtration comes from lighting which anc vary the image over 10 R, but film varies only about CC02 over that range in any given direction. So, the film is very stable.

Current papers are also withing that same range of CC02 and these two limits are at the range of human detection. Two people will just about see a CC02 and that is what is called a JND (Just Noticable Difference).

This statement applies only to Kodak papers.

PE

Photo Engineer said:

Enlargers vary in the hue of the light they output due to the optics and light source. Once you have the filtration for your enlarger, then any negative exposed in the last 50 years or so (except for Type CU Kodakcolor) will print within about a 10 R filter of that center point. I have over 1000 proof sheets to prove that point.

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This may be theoretically true, and it may even be true in practice if film and paper are processed under ideal laboratory conditions, but it's not true in practice of film processed at real-world commercial labs. I see quite a bit of variation in the filtration required for my negatives, including those that were processed at commercial labs. (I haven't checked to see if the results from specific labs are internally consistent, though.)

Photo Engineer said:

Wiggy;

I have said this before.

Enlargers vary in the hue of the light they output due to the optics and light source. Once you have the filtration for your enlarger, then any negative exposed in the last 50 years or so (except for Type CU Kodakcolor) will print within about a 10 R filter of that center point. I have over 1000 proof sheets to prove that point.

The major fluctuation in filtration comes from lighting which anc vary the image over 10 R, but film varies only about CC02 over that range in any given direction. So, the film is very stable.

Current papers are also withing that same range of CC02 and these two limits are at the range of human detection. Two people will just about see a CC02 and that is what is called a JND (Just Noticable Difference).

This statement applies only to Kodak papers.

PE

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So, if I am correct in my understanding of what you're saying, if you could reproduce exactly the colour balance and temperature of the light source that the paper emulsion is calibrated to then all other things being equal you wouldn't need any filtration and the filtration we use is primarily to correct for the enlarger light source.

Wiggy;

If we all used the same light source, heat absorber and filter types, then our filter packs would be nearly identical if not identical.

As for the comment by SRC, I can say that I have been processing my own film for over 50 years using Kodak chemistry or having it done by Kodak, and the results vary by not more than 20 R in pack. I have coated Kodakcolor films and Ektacolor papers, and can verify that the aim speed tolerance is about 02. This is even stated by Kodak in a publication.

The balance for all Kodak color negative films is daylight with speeds matched at 5500 K (IIRC). The paper is matched (or was) at 3200K IIRC. Negative films are as closely controlled as reversal films as to release speeds, or the final result would be ungood.

The story for papers is poorer. It was not until 1970 that Kodak was able to stabilze speeds of color papers with the new emulsions, but since then speed and balance have been just about spot on.

As I said, I have printed 1000+ proof sheets from 10 boxes of Endura at the rate of about 20 - 40 8x10s / night for months. I used the same filter pack, exposure time and lens opening. The variation was within the expeced value of about 20R and that was due mainly to lighting such as blue flash, electronic flash, tungsten mixes, fluorescent, morning, afternoon and evening and reflections from the surround. The worst results were from some Gold 400 shot by a wedding photographer for one of our daughters. They were rather blue but right on for exposure. I would guess it was about a 10B off from the norm. Kodacolor CU film from the 60s was balanced half way between daylight and tungsten. It was awful and a total failure. These do not print within the norm.

If you wish, I can let you page through the 1000+ sheets of my portfolio and judge for yourself. It is here in about a dozen 3" binders for anyone to see. The shots are divided into camera type / film size and so I have 35mm books, 645 books, 67 books and 4x5 books, all color neg from Kodacolor C-22 up to the last version of Portra 160VC.

Stop by.

PE

Photo Engineer said:

As for the comment by SRC, I can say that I have been processing my own film for over 50 years using Kodak chemistry or having it done by Kodak, and the results vary by not more than 20 R in pack.
...
If you wish, I can let you page through the 1000+ sheets of my portfolio and judge for yourself.

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I'm not questioning your truthfulness; I'm simply pointing out that the degree of control you yourself have attained, and that Kodak presumably attains in its commercial work, does not appear to be matched by all photofinishers. In the real world, if you've got multiple rolls of film processed by different labs, the filtration required for consistent color will vary, at least if my experience is any indication.

In other words, you said that a thing is so, and I'm saying that it can be so, but will not always be so.

SRS;

Remember that I was director of still photography at Cape Canaveral for 1.5 years, and I can say that their results (process) match mine, and I can say that the photofinishing lab that I worked in through College got similar good results as did our own tests.

So, the bottom line is that it can be done and I have done it at home, in my workplace, at Cape Canaveral and at Kodak. Now, the big difference is that if I used second source chemistry or messed up the process I got results quite at odds with what I've been describing.

Many labs don't run test strips or skimp on replenishment, or they use second source 'whatever' brand chemistry. A big fault is using blix or bad blix which can cause serious problems.

IDK what the problem is, but I know that I and my friends can repeat our results. BTW, I taught this at the Kodak Camera Club for a year as well.

PE

I think that the problem here is that since a negative cannot be viewed for quality unless one is a very experienced person in this field, most labs just 'pass' for processing. Also, for economy, many labs use low end kits, not the real thing.

I always use Kodak chemistry and keep a tight control to insure good results that meet the criteria I know have been set at the plant.

I don't doubt that people get things that vary, but if it was a slide you would complain. If it is a negative, they can 'print around it'. I think that is why our answers differ.

PE

I too can vouch for the control and consistency of Kodak colour negative film.

I have been doing my own colour neg processing at home for the last 20 years, prior to that I used a commercial lab (where I worked) to process my colour neg film. The commercial lab was a Kodak lab and we followed Kodak instructions to the letter, as well as their quality control systems.

My Kodak colour negatives can be seen to be extremely consistent by viewing the contact sheets I have. These contact sheets are made from every film for enlarging purposes. Almost all of the contact sheets have the same filtration, I know this as when doing a set of contacts I dial in my contact filtration and just fire away. The colour filtration hardly changes from film to film and from different light sources, except for certain fluorescent and sodium lamps.

There are of course differences, but by and large the contact sheets are very close to correct colour. From these I can glean the necessary information regarding density and possible colour corrections required. I know that if I cannot nail correct density and colour on the second print I'm disappointed with myself.

I do have a colour analyser, the Jobo Colorstar 1000, it hardly gets used and has hardly ever been used. In the commercial colour lab that I worked in, we had no colour analysers at all, what we did use, were heaps of the little Ilford enlarging meter, every enlarger had one.

One of the problems with colour enlarging is that it can be quite hard to get your thinking around correct filtration and correct density. I do know that one day you will eventually have a switch go off inside your head and bingo, it starts happening. Best of all, you then know how things happen and why.

Regarding non Kodak colour neg films, I don't believe they are (or were) as consistent as Kodak films. Fuji Realla, which I have used since it's release, is a wonderful low contrast film, or was. Over the years I have noted three major changes of colour filtration with this film, which suggested to me that when Fuji did do a major tweak to the film, the colour formulae were seemingly altered each time. I don't know what it was as I'm only a photographer/printer who takes copious notes, but in the life of Realla film up to about two years ago these are the differences I have noted for colour printing.

If you are printing and the colour starts to go off, it could be telling you your lamp is about to blow, density differences in conjunction with colour requiring correcting are the first signs of this.

Mick.

I'm another poster who has not had a problem with consistency of the C41 process.

Only 5 years experience processing C41/RA4 but it has always printed within 20 CC units of my baseline.

I stick to C41 process temps, but do cooler RA4 without a problem. I just use an old Kodak colour correction filter set to eyeball the corrections.

I have more problems if the bar fridge kicks in (it stores the stock) in the middle of an exposure.

I've not had a problem with Reala, but I tend to use a lot of this in 120 so buy in bulk.



Graham.

P.E. I own a small Pro Lab in sydney australia which runs kodak chemistry. I stick to Kodaks guidelines replenish at the right rates and run controls twice a day. I couldn't agree more with what you are saying. And as Mick has said I too can vouch for the control and consistency of Kodak color negative film.

Photo Engineer said:

Wiggy;

... If you wish, I can let you page through the 1000+ sheets of my portfolio and judge for yourself. It is here in about a dozen 3" binders for anyone to see. The shots are divided into camera type / film size and so I have 35mm books, 645 books, 67 books and 4x5 books, all color neg from Kodacolor C-22 up to the last version of Portra 160VC.

Stop by.

PE

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I'd love to but regrettably it might be a bit difficult as you're on the other side of the pond (did remind me that I'd not updated my profile!) Next time I'm in NY, poss sometime mid 2008, I'll give you a call.

Photo Engineer said:

I think that is why our answers differ.

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With the extended discussion, I don't think that our answers differ that much. I responded mainly because of this sentence in your earlier post:

Once you have the filtration for your enlarger, then any negative exposed in the last 50 years or so (except for Type CU Kodakcolor) will print within about a 10 R filter of that center point.

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That statement ignores processing. It may be true for negatives that have been optimally processed, but it's not true in the real world -- it needs qualifications concerning the nature of the processing. Because I was a teenager with little income in the 1980s, I used a lot of cut-rate photofinishers at that time, and a variety of somewhat less cut-rate but non-Kodak photofinishers in the following couple of decades. I've therefore now got a lot of negatives that were processed by at least a dozen different labs, on film from five different manufacturers. That's probably typical of consumers who shot film over this period and actually kept the negatives. I can get good color from these negatives when printing today, but the filtration values are all over the map. I can certainly believe that if I'd used Kodak or other higher-quality photofinishers through this whole period I'd have more consistent filtration today, but that's not the reality I face.

I understand your point and don't disagree.

I have used Kodak, Agfa, Sakura and Fuji films, all processed in Kodak chemistry or in the proprietary chemistry of the respective companies. Almost all of these print well within the specs I've given above, and therefore I must conclude that the processes are the culprits.

Of course, Agfa is the worst and has awful keeping as well so it falls outside this range in many cases. Other than that, the rest are very well behaved.

It seems that others have observed what I have as well.

PE

Wiggy;

Hope to see you in '08, here in Rochester. I'll show you the proofs and also give you a demo of emulsion making and coating if you wish.

PE

PE and others:

Do you ever use different filtration at the enlarger, to correct for colour casts arising from the light source? For example, a blue cast, due to open shade lighting?

Matt

MattKing said:

PE and others:

Do you ever use different filtration at the enlarger, to correct for colour casts arising from the light source? For example, a blue cast, due to open shade lighting?

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Speaking as one "other," yes, I do this. The most extreme case for me has been correcting for tungsten lighting on daylight-balanced film; however, photos shot on the same roll in more subtly different lighting (shade vs. direct sunlight, for instance) also requires adjustment. The latter is much more subtle than the tungsten/daylight thing.

MattKing said:

PE and others:

Do you ever use different filtration at the enlarger, to correct for colour casts arising from the light source? For example, a blue cast, due to open shade lighting?

Matt

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Yes. Changes in Colour temp of the scene and differences in exposure (over under just right) will change the filter pack from one frame to the next let alone one roll to the next. Adjusting for tungsten on daylight balanced film will often need a 45cc change (30cc yellow and 15cc mag as a starting point) from what you'd expect under daylight. Length of exposure will also will require filter pack changes and subjectivity can require still further changes.

I take notes on the backs of my contacts and use those numbers as starting points for new work. I do find that one film/paper combination will be in similar ranges, and the portra films are remarkably consistent especially on Kodak papers, but when you go from Kodak to Fuji or Agfa there are more often than not large swings in filtration. Older Kodak films also seemed to deviate a good bit as well PRN, PRT, VPS, VPL, PMZ and PMX were nowhere as consistent, but my memory could be fuzzier here.

I must say 20cc is a fairly large range and therefore not too useful as a bit of knowledge. It is so large that I wouldn't have found it as being noteworthy—I certainly never noticed and I am taking Ron’s word for it. It’s kind of like saying cook the bread for 20 to 30 mins at 250 to 350F.