Hi, possibly stupid question:
These are screenshots of two frames from the same roll of film, shot with the same camera and lens (Canon EOS 3 and EF 35mm 1.4L ii, fwiw). The difference i’m confused about is one has quite a hard, defined edge on the image. The other is mutch fuzzier. Anyone have any idea what’s causing this?
(ignore the fact that these are different corners, and one of them has the crop interface open 
)
Negative edges are always a bit fuzzy (same with transparencies) and if the lens has some field curvature, edges get more or less soft, depending on where the focus was set.
And they are sharper, when you have stopped down.
This puzzle cannot be solved with the information you provided.
Is this an ongoing problem or a single occurrence?
Are these contiguous frames?
Does this happen at the beginning or end of the film roll?
Was the lens removed and replaced before the second shot?
I could ask more questions and TOTALLY understand the need to figure this out, but I don’t see how this is a problem for the final image. Crop tighter and remember to overframe future shots.
The most likely answer is your camera is mechanically failing.
EDIT: I did assume you were talking about a film camera and these problems show up on the film negative. I have no experience with Canon DSLRs and see you are probably digitizing existing film with the EOS 3 and this defect is not visible on the film.
Isolate where the problem happens from the film holder to the lens, extension tube, and camera mount (is the camera mount parallel to the film and locked down). If the problem cannot be repeated, you will never find an answer!
I think @Ger has it correct - the images with more defined edges are those with stopped down edges
@222dfb you understood it correctly the first time- these artefacts do appear on the negatives. I do not think the camera is failing, I think this is just the lens’ image circle is more pronounced outside of the 35mm frame when stopped down - I am in no way worried. This fuzziness appears to be “extra” image rather than inside the image anyway