Hmm, I think I need to revisit how to extract Apple Photo's processed version of an unmodified raw - using dcraw to process a raw not only leaves areas that should be cropped, it doesn't do any reasonable brightness correction. But on the plus side, you get to see something closer to what the camera actually stores, including this vignetting from an f8 exposure, owing to the ridiculous brightness there.

Well, I guess the only way to work around it is to make some kind of adjustment in Apple Photos. Oh well :-/