RawTherapee, darktable, and other programs use lensfun for lens correction. If your lens appears to be absent from the database...

1. Check the Lensfun coverage database to make sure it really hasn't been added yet. If it's already there, you may need to update your local copy (sudo lensfun-update-data).

2. If your lens is not supported, you can read about how to calibrate it:

   • Torsten Bronger's Lens calibration tutorial

   • Adding new lenses to the database

3. If you can't (or don't want to) figure out how to calibrate your lens (it's not easy, and this is the approach I've taken in the past), you can send calibration images to Torsten Bronger. He will send the calibration data to both you and the lensfun maintainers.

junkyardsparkle notes:

... Make sure that what you want to correct is actually correctable with a lens profile. There seems to be some confusion about this amongst people submitting phone images in particular. The RAW files from my S6, for example, are already quite rectilinear.

Also, calibration images must be in RAW formats for RAW-capable cameras because many modern cameras (eg, Fuji) automatically correct lens imperfections when generating JPEGs.

4. If you can't (or don't want to) take calibration images or wait for a large upload to complete, some programs allow lens correction parameters to be entered manually. You may be able to find, by trial and error, parameters that you like.