I'm using LR 6.0 because I don't want to pay a monthly subscription. It could read the Canon CR2 files of my Canon 7D but isn't capable of reading the CR3 files of my Canon EOS RP so I used Adobe DNG Converter to convert the CR3 files to DNG.
Any images shot at the full-wide, 24mm lens position (RF 24-240mm F4-6.3 lens on EOS EP body) have extreme vignetting in the converted DNG files. Even if I go under Lens Correction, select Manual (there's no profile for my lens under Profile) and select the maximum +100 Vignetting correction, there's still significat vignetting.
This vignetting is being introduced by DNG Converter in the conversion process. If I open the CR3 files in Canon Digitial Photo Professional, there is no vignettingin the original files. The vignetting only occurs in photos taken at the widest, 24mm position. If the shot was zoomed even a little, there's no vignetting in the converted DNG file. I'd have to take a series of test shots to see exactly what focal length the vignetting disappears though.
Here's a screen shot of photos with and without vignetting taken at 24mm and more zoomed focal lengths...
While the shot above may look like lens vignetting distortion, I think it's actually being introduced in the CR3->DNG conversion process by Adobe DNG Converter. Here's the same photo side-by-side in Canon Digitial Photo Professional in original CR3 version and Adobd DNG Converter converted DNG version.
Can someone help my understand why DNG Converter is introducing vignetting in the CR3-to-DNG conversion process when the vignetting is not present in the original files?
Looks more like your lens hood was on improperly. That dark, more likely the hood.
Been their, done that.
Look at images in camera to verify/disprove.
The wider the lens, the more prone to the hood getting in the way..
Based on your side-by-side comparison, it is obvious that the Canon software is cropping the image to remove the vignetting. You can manually do this in LrC.
Is this a crop-sensor lens being used on a a full-frame body? It would seem that the full-sensor is being used to record the pixels while the APS-C crop info is being stored in the proprietary EXIF data that LrC is not reading.
Based on the above explanation, which I agree with, this doesn't seem to be a Lightroom problem at all.
I agree with all answers given above-
This is Lens Hood intruding into the photo.
You would notice the 'sharpness' of the dark corners change with different apertures set.
It's clear from the side-by-side images (final image above) of the boardwalk that the CR3 is cropped while the DNG is showing the entire image captured by the sensor.
I suspect what you are seeing is not related to the DNG conversion but it actually one of the limitations of the RF 24-240mm at 24mm. This lens does not come with a lens hood but it does display mechanical vignetting at 24mm. Canon corrects this in DPP.
This is discussed by Dustin Abbot in his review of this lens (refer "RF24-240 Image Quality")
If no hood on lens. Did you have a filter attached. Thick enough of a filter, or filters, then possable corner issues at the wide end.