I’m not sure if there are any negadoctor experts out there other than @aurelienpierre, but I thought I’d pose this challenge to give you all some fun and to help me out with my workflow. Many of the negatives I’m scanning were taken decades ago on a budget camera with poor photography skills. But I still want to keep them for posterity.
This photo was taken in South Africa in harsh midday sun, so there is strong contrast and washed out colours. The composition is pretty poor with a random bit of bench occupying the bottom right corner. But it poses a challenge for negadoctor and colour correction, so I thought it would be a good one to submit. Read on for my own workflow and some questions.
Kirstenbosch Neg 35.CR2 (23.6 MB)
My attempt with darktable 3.6:
Kirstenbosch Neg 35.CR2.xmp (13.8 KB)
This file is licensed Creative Commons, By-Attribution, Share-Alike.
Here is my workflow with some questions added:
- First, I turn off anything automatically applied (filmic RGB and automatic exposure adjustment)
- I then set color calibration to my LED light source temperature (4998K). I had previously taken a picture of the light source and taken a spot reading.
- I enable negadoctor and use the film base color picker to sample the unexposed film. At this stage, the white balance seems way off.
Q: At this stage, should I be trying to adjust white balance? If so, is it best to do this in negadoctor using the D min sliders or in color calibration or somewhere else?
- I then straighten and crop the image. This ensures that the 98% bounding box of all the other color pickers in negadoctor will only apply to the exposed part of the film.
- Back to negadoctor and I click the color picker for D max (setting the white point). This brightens the image, but I notice that it doesn’t get anywhere near the 100% line of the waveform/histogram. As there are certainly bright parts in this photo, I feel it should be filling up more of the histogram. So I draw a new bounding box to just capture the brightest parts of the image. Sometimes dust and specular highlights can cause problems with white point setting, so a smaller sample of the image can mitigate this.
Q: Do I adjust the D max manually to push up the white point? Or do I wait until I’ve completed the other tabs in negadoctor to judge the dynamic range? If I adjust D max manually at this stage, the highlights compress but still don’t go near the 100% line of the waveform/histogram.
- In negadoctor, I click the color picker for the scan exposure bias. This sets the black point and it looks ok. The image still has a flattened dynamic range and the colors are way off.
- On to the corrections tab, I click the color picker for shadows color cast at default settings. I then click the color picker for the highlights color cast at default settings. It’s done a decent job of removing the green cast and the dynamic range has expanded, but quite a bit more needs to be done and a new blue cast has been added.
- On to the print properties tab. I click the color picker for paper black. The image gets a little brighter. I usually leave the paper grade and paper gloss sliders alone (as per the manual, adjustment is not usually needed).
- Finally, I click the color picker for print exposure adjustment and another automatic adjustment is made. The image gets a little darker again. For this image, I personally don’t notice much difference after going through the properties tab, but it does depend on the photo I’m working on.
- I’m still left with a flat image and a strong color cast. So, I obviously need to adjust the tone curve to expand the dynamic range and then do some heavy colour correction work.
This is what I come up with after lots of work in color balance RGB and also applying local contrast and some sharpening.
I feel this final image is a good representation of the original print. The problems that remain are all from the original capture and conditions. So overall I think darktable has done a fantastic job.
But I guess my main question can be summed up as:
“At what point and where is it best to adjust 1) white balance and 2) dynamic range?”
I’m not always sure when to do these various adjustments and whether they should be done as much as possible in negadoctor or to leave it to other modules like color balance RGB, color cablibration, etc.
Update for anyone interested: I am now using a second instance of color calibration above negadoctor in the pixelpipe and performing another white balance adjustment after negadoctor. This is great for fine-tuning the white balance. The manual supports this special use case and the warning that CAT has been applied twice can be safely ignored. I’ve turned off the warning in preferences. Tonal adjustments can also be done with tone equalizer as long it is also placed above negadoctor in the pixelpipe.