I am trying to determine the vignetting corrections for a new lens (Laowa
7.5mm F2.0 MFT) using using the lens_calibrate.py program and images taken
through a translucent sheet. I appreciate that with a very wide angle lens
like this it might overestimate the vignetting, but that is not the
problem.  I run the program and get the lensfun.xml file which I then add
to the lensfun database (there is already an entry there for this lens for
chromatic abberation and distortion). If I then use darktable to apply this
correction to either the .pbm or .pgm files produced by lens_calibrate the
correction is very good (as it is for a 16bit tiff exported directly from
the demosaiced raw without any other processing)  However if I apply it to
the original raw file (Olympus .orf) the correction is very poor with only
about half the degree of correction required being made (though it does
seem to be consistently out by proportionally the same amount).  Thinking
this might be a darktable problem I then tried rawtherapee and ufraw, but
they both had the same problem with the raw file.  Does anyone have any
ideas what might be going on, or has anyone seen anything similar?
Obviously I will raise this with darktable developers (my preferred
application), but given that rawtherapee and and ufraw behave in the same
way there may be a generic problem.

regards,

Jim
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