This is another question arising from my slide digitising project but it seems equally applicable to any photo taken through, shall I say, ‘custom’ optics or with less than perfect illumination… basically I mean situations where off the peg lens corrections are not available. In my case the issue can be from the backlight and also from light fall off in the optics of the camera that took the original image.
Searching around this topic turned up a suggestion to use the vignetting module. I spent some time playing with the sliders on an image of my backlight diffuser and managed to get the luminance (checked with the colour picker at various points around the image) nice and even.
Making that into a style and applying it to a real photo produced very disappointing results. Firstly the brightness at the edges was far too much - easy enough to change but having done that the sky looked good but everything else was washed out towards the edge of the frame.
I guess I could make a parametric mask to limit the effect of the vignette to the sky but is there a better way?
Taking a lesson from another field… in astrophotography we take ‘flat’ frames to characterise the variations in light transmission across the frame. These are then used as part of the stacking process. Is there a way to use a ‘flat’ frame in darktable?