It is inevitable that wide-angle photographs (taken towards the shorter end of this zoom range) will show increasingly "unnatural looking" towards the edges and corners. This is by and large a viewing problem. The original scene looks natural to us (from a certain viewpoint) and if an (undistorted) photo of the scene was instead held up in front of us at the right distance from our eye for however large that photograph was, that photograph would look essentially the same as the real scene does. This hypothetical photo, in this special circumstance, "mapping on to" - lining up with - what we otherwise see when the photo is not sitting in the way..
The unnatural look of a wideangle photo happens in a circumstance where a photo of the scene is being viewed either further away, or smaller, than that. A fully corrected, undistorted - in the sense of "rectilinear" - wideangle photo can look like a completely natural representation of the scene, but with a typical display - or print size - it is hard to get the eye close enough and still focus on the image. But one can get some idea.
If one imagines scaling this viewing circumstance up - with the image printed ten feet wide, and viewed from a more comfortable distance - one can better imagine how the photo is genuinely "mapping onto" one's real-life view of the scene. But only when seen at that distance, from that viewpoint.
The same effect is present in ALL viewing of photos, captured with all kinds of lenses - but much less noticeable when BOTH a moderate focal length has been used AND a moderate viewing size and distance are being used. That is part of the reason we would consider a given focal length to be 'moderate'.
There are various aesthetic and compositional tricks to make this 'wideangle effect' less pictorially objectionable - there is also a converse 'telephoto effect' for long focal lengths. We are more used to seeing the latter and tend to mind it less. Or there are other tactics still - such as deliberately NOT going for a perfect rectilinear projection, but rather accepting a somewhat "fisheye" one. But since a rectilinear expectation has been built into the lens design - unless that lens is explicitly a "fisheye" one - trying to do this in software is a bit of a forced compromise, with some downsides. To play, one can try the Manual tab in Lens Corrections where there is a "distortion" slider - in sample below, I also disabled "constrain to crop" and used the Scaling in Transform panel to expose better what is being done to the image.
This photo was taken at 12mm on an APS-C sensor, the same as roughly 18mm on your camera. Unlike the rectilinear version, it cannot be viewed from any distance that would map onto one's natural view of the scene - when shown as a flat surface.
But one can imagine printing it onto a curved surface instead, which then could map OK with the scene as seen (grin) - but with viewing distance then becoming even more critical of course.
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