Let me tell my view.
IMHO the critical process is at exposure time. Then you take spectral information hitting the negative in each image point and you convert that SPD in each point to 3 values. After doing the 1st development of the C-41 you get in the negative 3 values of silver density, one in each of the 3 color layers. This is where true metamerism takes place !!!! In the same way a digital sensor converts the spectum to 3 RGB voltage levels in the bayer pixels. Also the human eye takes the spectrum hitting a point and makes a phototransduction (
https://en.wikipedia.org/wiki/Visual_phototransduction), RGB chromophores in the RGB cones filter the spectrum and that ends in "3 electrical signals".
Kodak and Fuji publish that critical data, Portra 160:
Then you make the color development stage, and silver is bleached. Those coupler dyes only work like an ICC profile, no information is lost at this stage, spectral information was lost (reduced to 3 exposure values) at the exposure time, the dye coupling only cooks the metameric information for a nice result, that's also linked with the RA-4 particular paper interpertation.
What I say is that there is no universal calibration than would make Velvia 50 portraits look as if Portra 160 was used, because the captured information has reduced spectrums to 3 color levels.
In the same way when we have the negative developed we are virtually playing with the metameric conversion done, the spectrums in the negative are from 3 overlaped curves that can be higher or lower, but always having a similar shape, depending on the coupling dyes the film uses, so at the end we have 3 virtual levels for each point.
We are not playing anymore with spectrums hitting every image point, but with reduced metameric information.
IMHO we have 2 cases.
a) Slides are a reference medium. A calibrated hybrid process should deliver an sRGB file that shown in a calibrated monitor would be as close as possible to the slide viewed directly. Here we have a problem because some Velvia tones cannot be seen in a monitor. Also the Velvia slide can be 3.4D while a monitor usually has 2.0D static contrast (and 200000000000000:1 dynamic contrast
)
b) C-41 is an industrial process intended to end in a print, but we have a necessary post-process to adjust the result. So IMHO for an scanned color negative we haven't a reference. IMHO single choice we have is selecting a nice reference for each film type. This would be taking sound prints from common shots and making the calibration from the print, or from a good digital edition of the scan, adding perhaps an smart equalization.
A C-41 negative to positive conversion could have some options: Standard, Landscape, Wedding José Villa, Vivid, Soft...
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