The tool I’m using to “scan” my negatives is a Sony A7R II camera and a color adjustable LED light source.
I am adjusting the color & exposure so that unexposed film records at about 90% full scale for each of the R, G, and B channels (yes, this needs to be different for each type of film I run into). This gets me the best color resolution, but I think it confuses the Rawtherapee tool I am using to convert negatives to positives.
I have had to turn off the white balance as it was getting wildly wrong results.
The reason I have been having to tweak every single photo is that the “copy and paste” of the processing of one photo to another gives wildly varying results. I always capture a bit of the unexposed border of the frame which should be black by definition. I adjust the R, G, and B channels to be the same brightness at the histogram peak for this unexposed part of the picture, and near the bottom of the histogram scale for one picture, copy the processing to the next picture, and find that the entire peak has shifted off the bottom of the scale, or to the middle of the histogram, and the three color peaks have separated significantly.
The exposure and aperture for all of the pictures is the same for all the pictures, and I would expect the color and brightness of the unexposed film to be the same for all frames.
I’ve tried to turn off all automatic adjustments, but obviously I’m missing some. I select “neutral” profile, turn off white balance, then enable the negative processing. I generally adjust the three exponents in the negative processing, exposure compensation, black level, lightness, contrast, saturation, and use the channel mixer. I’m not particularly happy with the results I’m getting, but I’d be much happier if I could copy the processing from one picture to the next without it being wildly altered.