Nature of color errors in off temp C41

[Nate Weaver]

Hi,

I dove headfirst into C41. I probably shouldn't have, but I did, so here I am. I have a not short (~20 years) professional history in motion picture film production, and then now digital cinema. I've been color correcting video/digital motion picture for almost 10 years now on my smaller jobs (DaVinci Resolve, which is a tremendous color toolset). Bigger jobs go to real colorists, but I know quite a bit about how to diagnose fix broken color.

Dove into a Pentax 67, shot Portra 160/400, home developed (with a Rollei Blix kit), bought a scanner, and having a hard time getting nice images.

I probably would have NOT dove into C41 if I would have understood what goes wrong with off temps, since I care about color rendition.

So my question is this. I have read numerous times on this board in multiple threads about "color crossover", and my first interpretation of that term would be (Scenario 1), say, the magenta dyes responding to the yellow layer halides due to too much pre-soak (too much developer infusion due to too long water immersion, say), or the cyan layer responding to magenta halides.

This would create very deep color problems indeed. It's what I'd call "channel mixing", and in my experience it can't be corrected effectively ever. To my knowledge it's destructive.

Later, I read an interpretation that had "color crossover" was more of just (Scenario 2) differing response curves in the CMY layers. Which is a much more straightforward problem to correct (at least in the digital world, if not on an enlarger). Magenta in the toe, yellow in the highlights, I can fix that effectively and precisely.

With the terrible, terrible scanner software available that is giving me wild color swings with my bad processing, and scant tools to correct them in the scan, I'm questioning a little what I'm up against with my dodgy processing.

So am I dealing Scenario 1, or Scenario 2 with off temp/time C41?

The image attached appears to only have yellow whites and magenta shadows, but the trim color of my house is off enough to suggest "color channel mixing". But it's not HORRIBLE.

Thanks in advance for any thoughts

-NW

[side note, the color correction tool I am most familiar with, DaVinci Resolve, was designed at the beginning to color correct film being transferred to video via flying spot telecine. So it really does have the right tools to correct out of spec negative film. Lift, Gamma Gain wheels make common problems a breeze. Lucky for me it can do 16bit TIFFs]

 

[Mr Bill]

Hi, I can't answer precisely, but I can point you at some things. At first glance, it would appear that your scenario 2, simple contrast differences in the colored dye layers, would be the predominant problem (when changes are made to film process temperature, time, etc., the top film layer tends to be affected the most, etc.). But... films like Portra do other little color-correcting things along the way - they have things called DIR color couplers, that when released will inhibit further development, perhaps in an adjacent dye layer. Or between certain layers, certain chemical scavengers might "mop up" chemicals trying to cross between color layers, etc. So this sort of thing, when disrupted, falls in your scenario 1, "channel mixing." I wouldn't expect either of these things to be TOO severe, as long as you are close to the aim temperatures. But I don't really know.

The next thing is the scanning stage, which is off-topic for Apug (sorry). But your scanner likely doesn't see the film exactly the same way as the paper does, so needs certain corrections. Although the film's orange mask and the DIRs, etc., mostly correct the film for optical printing, your scanner probably needs to use a corrective "matrix" in the software, it ought to come from one of the manufacturers who knows how to figure it.

If I were in your shoes, the first thing I'd do is to evaluate my film processing. In commercial processing they'd run special test strips, aka "process control strips" supplied by the manufacturer, then read with a densitometer. In your case, what you might do is shoot a double set of test images, then have one roll processed by a trusted lab (you presume that they are in spec); you do the other roll yourself, hoping that it's close to a dead match.

If you want to get a rough idea of what various process errors do to C-41 film, look for Kodak's Z-131 manual. The last part, on process monitoring, shows how the control strips behave under certain conditions. If you're familiar with the "characteristic curve" of film (aka H&D, or D log E, D log H, etc.), the control strips "LD" point is fairly low on the curve, and the HD point is fairly high. So when they plot "HD-LD," this is a measure of contrast.

Best of luck. This is certainly no turn-key operation.

 

[Sirius Glass]

Welcome to APUG

I have developed much C-41 film without problems, but I carefully stick to the recommended temperature. Photo Engineer would be the best person to talk to at APUG or anywhere.

 

[chassis]

Hi Nate, welcome! I'm seeing some overexposure, and shadows exposed by blue sky. Remove some blue using curves and bring all of the values down. Then see how you like it. I am not seeing chemical/processing errors here but others may differ. I agree with SG that sticking to Kodak's C-41 parameters is the best way forward.

 

[pentaxuser]

Well Nate only you know the exact colour of your house but it may be the case that inherently the film's colours do not quite match the shade exactly. I believe for instance that it is said that certain films do greens better than other films. There is relatively little in the way of whites in you attachment but the clouds and the white signs look OK to me.
In fact I can see nothing wrong with the attachment but I may be easily pleased

pentaxuser

 

[Photo Engineer]

Actually, both scenarios are the same when you get down to it. All three layers must have parallel curves if you shoot a neutral, and the curves then for single colors fall where the design parameters have made provision for - if you process correctly. If you have crossover, either 1 or 2, then a neutral might go from pink to green (either way - up or down the scale). It can never be corrected.

And, BTW, all layers affect each other all of the time. That is one of the parameters.

Thus, all C41 products must work at 100F and complete development in 3'15" for proper results. A prewet is not a problem. I use it all the time.

PE

 

[Nate Weaver]

I appreciate the responses. I suspect the truth is I'm close to being on the mark, but a little off on temps and all over the place on exposure (I'm using a iPhone app that purports to do something like matrix metering, but learning it falls down). Add in typical lighting/real life color issues (skylight vs. direct light, etc) and it becomes a little tangled for me.

[edit: I was composing this and then Photo Engineer posted]

Thanks PE for chiming in. It took about 4 readings of your post to parse how your statement about "scenario 1 & 2" (you're all so awesome for accepting my presentation) essentially being the same, but now I think I've wrapped my head around it. At the end of the day (or at the end of the bad process!), all you have is mismatched response curves. I was trying to differentiate possible causes for such a thing, but they both have the same causes (bad temp/time), and same result. So who cares.

I suppose the easiest way to get some perspective is just shoot a control with same film and subject and get it pro processed. Of course that's what I'm missing right now, experience and perspective.

Again, thanks all. I think my situation (i.e. home processing) is not as bad as I hypothesized.

 

[chassis]

Nate, it sounds like you have a fair bit of imaging experience and the idea to shoot a control is good. I made a home made color checker using paint chips from the hardware store, pasted onto an 8.5x11 black card. I shoot the checker for client work, or critical personal work, in each lighting situation, if there is more than one (daylight, strobe, etc.). It helps me tremendously when I am postprocessing.

The image was not 100% postprocessed so has dust, imperfections, etc. Kodak Portra 400 through an RB67 Pro SD with Sekor C 180/4.5. Yes it's overexposed, my strobe metering workflow needs tweaking.

colorcheck by Wilmarco Imaging, on Flickr

 

[Photo Engineer]

The above example shows a large amount of crossover. It goes from light cyan to light red in the neutral.

PE

 

[Prof_Pixel]

The above example shows a large amount of crossover. It goes from light cyan to light red in the neutral.

PE

It could be just an issue of using paint chips; the actual MacBeth charts use carefully monitored materials.

See https://en.wikipedia.org/wiki/ColorChecker

 

[Photo Engineer]

Hah, I missed that one. Thanks Fred.

Yes, sensitivity to paint chips can cause problems like that. However, I did take into consideration the greenish shadows on the fingers and collar.

PE