ALTSON, is that the name of the adapter? If that is the case, then you may need to copy the lens data for 50/1.4 and rename the new part as ALTSON+Canon EF 50mm f/1.4 USM instead of Canon EF 50mm f/1.4 USM.
Yes, that works, but I noticed that the parameters are not remembered by darktable and the correction is lost when the file is reverted to its original contents. I wouldn’t like to depend on modifying the file on every lensfun update, and I wouldn’t like to depend on a local copy being present in ~/.local/share/lensfun (which didn’t work under Flatpak anyway), so this is not an acceptable long-term solution for me.
On the up side, while editing the file and looking around, I noticed there is a number of “Generic” cameras which are compatible with all lenses. Once I chose the “Generic, Crop-factor 1.5 (APS-C)” camera, I was able to find the Canon lens in the lens selection menu, and that works well enough for me. Thank you for the pointer!
Sure, here is one, with the XMP demonstrating the use of the “Generic, Crop-factor 1.5 (APS-C)” camera: DSCF8523.RAF (31.9 MB) DSCF8523.RAF.xmp (6.6 KB)
Yes, ALTSON is the brand name on the adapter. I do not mind picking the lens manually from the drop-down, but thanks for the tip!