I was just curious if DT has a module that will correct pincushion or barrel distortion when the lens is unknown or not supported in the lens correction module? If not, is it a feature that would be worth requesting?
I think best would be to measure the lens, determine the correction parameters, and put them in the lensfun data base. There’s a way to add your own file in a location that doesn’t get overwritten by database updates.
I guess what I am looking for or suggesting is a lens distortion feature like this filter found in GIMP.
I’ve fallen somewhat out of close familiarity with darktable’s details, but is it possible to get Adobe LCP data into darktable (maybe via lensfun)? If so, you can use the Adobe LCP creator (Windows only) to profile your lens. I created a profile for a manual SLR 50mm lens and use that LCP file directly in ART. Dunno if that can be done with darktable, though.
If that’s not a good / possible idea just tell me to shut up. 
There are times we may have images from unknown lens and I feel it would benefit DT (or any program) to have manual adjustments like available in GIMP. Obviously I could just edit in DT and export to GIMP for the distortion fix.
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Recent lensfun has a utility, lensfun-convert-lcp, does what the name describes. You can read about it here:
https://lensfun.github.io/manual/v0.3.95/lensfun-convert-lcp.html
Since it’s in the 0.3.95 version of lensfun, it may do a version_2 entry, which won’t be read by 0.3.2, the stable version…
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I thought of adding that but dropped it after evaluation because of a) inferiour quality plus wanting to avoid an exploding UI and b) increase pressure on users to contribute to lensfun
I’d find that useful on occasion too.
True!
Trouble is, on the occasions when I’ve wanted to correct a lens that’s not supported, it’s usually been a one-off. I should still learn how to contribute to Lensfun, but it would be no help to getting my image sorted on that particular occasion. I dunno really…
That’s exactly what I do. I’ve lots of rare vintage lenses in my collection that no one’s ever heard of.
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How does that work? Lensfun relies on exif data right, selecting the lens is doable but corrections are aperture and zoom dependent so you’ll get the wrong settings?
Am I missing something? I should really profile my vintage lenses but haven’t done it because I see no way of applying the correct settings.
Thanks!
Thanks for the replies so far and thanks for the informative links. I see I can set a lens correction manually which is vey helpful. However, there are times when I have images or scanned images with no lens or camera data known to me, but there is pincushion or barrel distortion that needs fixing. Yes I can do this in GIMP so it is not a huge problem to me, but it would be nice to have additional sliders in DT to tackle these problems. The developers have given sliders for other corrections such as vignetting and TCA so I suggest slider/s for pincushion and barrel distortion would also be good.
Spot on. There are numerous ways of doing so, including this method; you can do something similar in RawTherapee, for example, or you can also edit the data directly in a text editor (if you’ve done the math; you can manually create a Lensfun database entry for manual lenses without exif data).
When I get a spare five minutes (probably never 
), I’ll put a tutorial up on my website (it’s been sitting in my to-do list for a while, actually, along with a million other things).
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