Manual lens correction

Software darktable
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I recently bought a Rokinon 12mm f/2.0 NCS CS (a.k.a. Samyang 12mm f/2.0 NCS CS) and used it for my first attempt at Milky Way photography. According to reviews, this lens is a great budget option for this kind of thing, and is supposed to not exhibit much coma. But when I looked at the photos, the brighter stars in the corners looked pretty bad:

20250504-061226-3396-_DSF4832_pixls
20250504-061226-3396-_DSF4832_pixls1620×1080 513 KB

Applying the Lensfun lens correction for Samyang 12mm f/2.0 NCS CS had approximately zero effect on the chromatic aberration. But then I enabled “TCA Override” and was able to eliminate the CA in a few moments, so I made myself a preset.

Before I go any further, I want to say that I am very impressed with how easy this was to accomplish. Kudos to whoever developed this support!

However, the odd shapes of the stars remain.

I have a couple of questions:

  1. My lens is supported by Lensfun, but I didn’t get good results without my tweaks. Should I submit samples? What is the likelihood that anyone is going to want them for a lens that is already supported? It could just be that I got a bad copy of the lens and the profile is fine for some other people with the same lens.
  2. Is there anything I can do in dt about the shape of the stars? This article and many others seem to suggest not. Maybe I’m just hoping for an unrealistic outcome.
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Did you try the different ‘target geometry’ options in the ‘lens correction’ module?

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I just did now. None helped to fix the stars, but all the options other than the originally selected rectilinear resulted in some pretty crazy images.

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4

Are all corrections supported?

If some corrections are missing, then they’d be very welcome.

For a lens this wide, doing pincushion correction will just make most of the frame soft.

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5

Yes.

OK, I’ll get on that, hopefully soon.

6

since there might be production variance you might profile your lens yourself (see Create your own lens correction data for Lensfun – Kameratrollet ) and use that instead of the lensfun provided corrections.

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