PLEASE NOTE!
This thread has been superseded by a later, updated and more-detailed tutorial in the Post-Processing Articles forum, here:
https://www.pentaxforums.com/forums/61-post-processing-articles/438373-colou...pee-5-8-a.html
I have asked the moderators to close this older, out-dated thread, and cordially direct you to the new one linked above for further discussion :)
Kind regds,
Mike
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IMPORTANT UPDATE!
Since writing this post and publishing the "how-to" video on YouTube, I've run into a couple of inaccurate conversions with some of my older negatives. Further investigation has revealed the cause (I'm a little annoyed with myself for missing it) and, as a result, I've refined the process very slightly:
When the raw file is loaded into GIMP - which in turn loads it into RawTherapee automatically - it's essential to un-check "Clip out-of-gamut colours" (the very first item in the Exposure section). If this box is checked (which is the default), it's possible (though not always the case) that the colours in the negative won't be true to what was captured by the camera, and the inverted image in GIMP may have a colour cast (the very thing we're trying to avoid!). So, please uncheck the box as directed, and it will work correctly!
I also recommend setting Saturation to +10 in RawTherapee before the Colour Toning step, as this ultimately results in better saturated positives - but you can always add saturation when conversion is complete...
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By now, quite a few of you will know that I'm shooting film and digitising my negatives at home with a DSLR. To convert them into positives, I've been using a commercial Lightroom plug-in called Negative Lab Pro. It generally does an excellent job - but it's not perfect, and can occasionally require a fair bit of tweaking or re-running to produce the desired output. It also hides what it does behind automation and presets (the whole point, to some extent), and this can make it difficult to predict results with absolute confidence.
I've tried Darktable's "Negadoctor" module, and RawTherapee's "Film Negative" module, and whilst they
can work pretty well, the results are extremely variable and highly dependent on the content of the digitised negative. I've also tried entirely manual conversion in GIMP, inverting the tone curve then adjusting R, G and B curves in a bid to achieve realistic colours and tones without any colour cast, but it can be very time consuming and rarely produces 100% satisfactory results.
For the past week or so, I've been working on a method of conversion using a combination of GIMP and RawTherapee, and I'm confident I've cracked it. I've tested the approach with numerous negatives of my own - plus a few belonging to others - and I'm getting
exactly the results I want and expect. It's more consistent and predictable than Negative Lab Pro, in my limited experience, and runs rings around Darktable and RawTherapee's integrated conversion modules.
The essential workflow is as follows:
- When digitising negatives, set the camera's custom white balance against the light source used
- Set correct camera and lens exposure parameters to ensure no clipping
- Capture negatives with at least some of the border visible in the frame
... then:
- Open a negative raw file in GIMP, which loads it into RawTherapee via plug-in
- In RT, uncheck the Clip out-of-gamut colours box (highlighted, because this is a new step as per the update at the top of the post ) - Optionally, set Saturation to +20 for more vibrant positives (or saturation can be added when conversion is complete, if required)
- Set a Linear / Standard tone curve
- Set White Balance to Camera
- Set the Input Profile to Camera Standard
- Use the Lockable Colour Picker tool to sample a point on the border
- Use the Colour Toning tool to adjust until the picker's RGB values match closely (within +/- 1%) - this effectively removes the film base colour at source
- Exit RT and the image is transferred to GIMP
- In GIMP, invert the image
- Select the exposed area of the photo and include a little of the border (but
no sprocket holes or other elements)
- Open the Levels tool and click Auto Input Levels
- Adjust brightness using the right-hand pointer of the Output Levels control (critical if the photo contains no highlights)
- Adjust gamma using the middle pointer of the Input Levels control (this varies photo to photo, and largely depends on the quality of the original exposure)
- Click OK to apply (don't click Cancel, or the levels won't be saved as a preset, which is needed for the next steps)
- Undo the adjustments (as they currently apply to an incomplete portion of the image)
- Select All
- Re-open the Levels tool and from the Presets drop-down, select the most recent "Last used" adjustments
- Click OK, and the conversion is complete
- Export the image as a positive TIFF master, with .TIF extension
This may seem like quite a bit of work, but having practiced variations of it countless times while researching this, it now takes me around 2 minutes from opening the file to exporting the completed positive master. I don't think that's bad for a manual conversion process where the user retains complete control, and one that produces consistently good results.
The great thing is, since this is based around GIMP and RawTherapee, it's completely free and will work for those with Windows, Mac and Linux systems.
The "secret sauce" in this approach is a recipe of three ingredients - (1) use of a relatively neutral camera profile, (2) a flat tone curve to begin with, and (3) use of RT's Colour Toning tool to cancel out the film base colour cast - much like acetate filters or a dichroic head in traditional wet-printing. It seems pretty obvious to me now, but removal of the film base colour
before conversion is absolutely key to achieving good colours in the final image, with no colour cast or shift to deal with.
I've put together a short YouTube video to demonstrate the process. I don't run a monetised channel, and I've no interest in building a community of followers. I may never produce another video
I'm not trying to pimp my own content and take attention away from my beloved PentaxForums... I simply made the video to assist others with quality colour negative conversions.
I'll admit, it was kind of fun putting together my first (and probably last) YouTube video... and I guess I'd get a kick if anyone clicks the like button on it
One in the eye for Jared Polin and Tony Northrup, eh?
Last edited by BigMackCam; 02-18-2022 at 04:03 PM.