I’ve recently acquired the Pentax 10-17mm fish-eye zoom and I’m interested in correcting for TCA whilst leaving the fish-eye distortion in place. However, after selecting the lens in the ‘lens correction’ module (it isn’t selected automatically, which I expect is due to a difference in the naming scheme), the image distortion/geometry is changed, even if I select ‘only TCA’, or even ‘none’.
There is a difference in the resultant image distortion between correction settings that do and don’t include ‘distortion’, but as mentioned, without ‘distortion’ the image is still drastically changed and at least somewhat “corrected”.
Before the lens correction module is activated:
After the lens correction module is activated, with ‘TCA & vignetting’ selected and all other module settings untouched:
I’ve tried playing with the other settings but don’t seem to be able to have this module activated while keeping the original shot geometry-wise. Is it/should it be possible? If it isn’t possible I can do without the lens correction module for this lens and stick to other means of getting rid of CA/fringing, but I thought I’d post about it in case I’m doing it wrong or it’s an issue. The manual seems to suggest that individual types of correction should be able to be omitted.
As an aside, based on one other test image it seems that the distortion correction is actually very good at getting rid of those curved straight lines, so if I do ever want to de-fish I expect it will be easy with Darktable. 
Semi-related, I see that the ‘distance to subject’ setting is for vignetting, but I don’t know what the unit it uses is. It defaults to 1000.
Thanks.
