Why is there color crossover in divided C-41?
After mixing the last batch of working solution from my Tetenal kit late last night, and facing having to buy another one when this stuff exhausts, I started thinking (never a good thing...). I had recently started working with Diafine in B&W, and have been impressed with its simplicity and flexibility. So of course, I thought, why not a divided developer? Wouldn't such a developer work for the first developer in C-41? And, as a divided developer, since it works to completion, doesn't that solve any crossover problems?
So some quick research on the web this morning showed me that in fact, this ground has been plowed before, by Patrick Dignan and by this guy: http://nelsonfoto.com/SMF/index.php?topic=9360.0
It seems that in fact, a divided development brew has been done, and it works at room temperature. This immediately sounds awesome to me, and seems a brew I can make myself, which is triple keen, assuming I can find the ingredients.
But then I read replies to Dave Lyga's post on diluted C-41. RPC reported that his sensitometry tests showed crossover in the negatives. Greg Davis showed results with images, but for a different alternative process.
I'm immediately concerned about crossover, because in my experience, it is not simple (or perhaps even possible, practically) to correct it sufficiently to get accurate colors across the spectrum. Tweak one color, you've lost another somewhere else in the image. Frustrating. I don't want it, I'm not a Lomographer.
So, my question is this, specifically for a divided development based process: why would there be crossover, if the developer works to completion in all layers? Wouldn't all the color layers be fully developed, and thus balanced? I am certain this question reveals my full ignorance of what happens in the C-41 process, which is the purpose of the post. Would those deeply knowledgeable about what is going on chemically respond? I would be curious to try a divided development process, but not if in the end I am faced with never ending crossover battles.
Last edited by chuck94022; 02-07-2013 at 09:25 PM. Click to view previous post history.
Reason: Misspelled Patrick Dignan's name
I would assume colour crossover from the interlayer effects not being present. Otherwise if they're present.. then it'd just be mismatched curves, not under correction of colour crossover in the masking (ie: Not the green portion having exposure from blue and/or red in it).
If you're correcting digitally, then a Xrite ColorChecker Passport will cure what ails you, but it's best of course to start off with the best source.
Add a few mg (1-1.4mg per litre - in reference to this formula - http://www.apug.org/forums/forum40/5...-near-c41.html ) of Potassium Iodide to at least part B (though you would likely want it in both parts) of Dignan's formula, skip the Benzotriazole. HAS is important in working solution developer as a preservative, and to keep replenishment rates low. So it may not be needed, though I would recommend some in part A to prevent excess fog. There are many suppliers of CD-4 in China. Check to see what acid salt they supply though, that'll change your mix ratios.
The Sodium Sulphite present activates at least some of the CD-4 in my experience (3.5g from calculation if using the H2SO4 salt of CD-4), iirc it's
H2SO4 + Na2SO3 -> Na2SO4 + H2O + SO2
98.079g + 126.043g
Some people skip the sulfite and find they got thin to no images. Having the developer activate only in bath B is not good enough, at least a thin image must form in Bath A for this to work reasonably.
You can replace the 4.5g of Sulfite with 9.5 grams of Sodium Bicarbonate to activate all CD-4 (H2SO4 salt), and then set the desired pH with sodium metabisulphite.
Last edited by Athiril; 02-07-2013 at 11:36 PM. Click to view previous post history.
Originally Posted by chuck94022
I think the simplest response is to say that no, the developer does NOT work to completion. If you doubt this, consider the situation of people who "push process" C-41 films. They use longer development times to increase the amount of development, which proves that the spec development is NOT to completion.
The development of C-41 films is an incredible balancing act between the processing chemicals and interactions within the emulsion, optimized for the spec conditions. So it should not be surprising that any variation might produce color crosses. I'm not sure how someone would measure these sensitometrically, except in the simple case of a neutral tone scale (photograph a gray scale, or the equivalent via a sensitometer). Even if no color crosses exist here, who can say if such would exist in a non-neutral case, such as human skin tones in portraiture?
I've done pretty extensive testing on pro color neg films for studio portraiture, doing evaluations for a large chain/finishing outfit. When new pro films were being introduced, we would start with sensitometric screening and simple shooting tests. These were followed up by exposure variation tests, where all would be optically printed onto the matching professional papers; the prints are hand-balanced to match one master control print, roughly to skin highlights. Professional color correctors, using special color booths, would evaluate the results, both critically and with respect to what an average person might notice. The results are too complicated to put into a written report, other than superficially, and certainly a densitometer is not adequate for this. If future questions should arise, we'd always pull the original test prints back out; maybe the test has to be repeated because of some other changes.
Those days are long gone, and modern digital printing can probably, in principle, be adjusted to correct nearly any deficiencies.
I would imagine you would shoot a grey scale along with various colour strips, and then you could measure the grey scale and compare to colour strips and see how much 'wrong colour' is in the colour strips (ie: green and blue in a mostly pure red target).
Originally Posted by Mr Bill
Well, assuming that nothing else is wrong with the method, you would still only see a certain relative strength of a certain color. For example, say that you used a strongly red-colored step wedge as a target. So perhaps you might say that an 80% saturated red color does not show a color cross. But isn't it possible that a 30% saturated red MIGHT have a crossover? Or a mix of 30% red and 50% blue, etc? The interactions between film layers are too complicated, I think, to generalize from a small handful of specific color tests.
Originally Posted by Athiril
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So if I understand what you are saying, Mr. Bill, the layers are not just individually developing, but they are also interacting with each other. Is that the case? Also, I think you are implying that since the layers aren't developing to completion, with the two bath development they may also not be developing at the same rate (and worse, perhaps development slows at different rates in different layers, causing crossover in some tonal values but not others). Am I understanding this correctly?
Athiril, the Colorchecker Passport is on my wishlist! Thanks.
Originally Posted by chuck94022
These effects seem to be very tricky. If you remember the introduction of Fuji Reala film, with the "fourth layer," they were able to counteract unwanted greenish effects under certain fluorescent lights. Later films seemed to handle this situation just about as well without a fourth layer. So some very tricky interimage effects are going on. These are commonly attributed to things called "development-inhibitor reactors," known as DIRs or Super DIRs. (I only know of these things from technical papers or the very occasional discussion with film engineers.)
I don't know much about the split developers so I shouldn't talk about them, but it is my opinion that modern C-41 film developing is such a finely balanced process that any tampering is more likely to unbalance things than it is to keep them the same. Just guessing, though.
To modify that: the emulsion design and developer design are, together, a finely balanced system.
Originally Posted by Mr Bill
Look guys, the C41 film family consists of over 10 layers with 3 sets of 3 emulsions making for a total of 9 emulsion types in each film. They all have to reach the right point at the right time or 3' 15" at 100F. Any change to that will cause problems (within limits of about 15" for push or pull). In fact, it is so critical that it is better to over and under expose than it is to push or pull.
And, correct processing is also a diffusion controlled process involving the color correcting chemicals which must diffuse properly to give the correct colors.
By comparison, B&W is a snap. Oh, and even in B&W a divided developer must be fined tuned for time and formulation for your particular film!
Anyhow, the guy in the reference from the OP posted the most difficult pictures to judge for crossover. For example, a portrait might have had greenish highlights and beefy higher density areas. BTDT. Anyhow, Pat Dignan and I exchanged letters on that in the '90s and we even talked on the phone. By the end, I think that I had him convinced that the divided developer idea might be a bad one. IDK for sure.
So, in a word, don't do it. Do it the right way.