Hm, no, I wasn’t doing any right-clicking on those icons. Actually, I wasn’t aware that you could change the focal length that way. Good to know.
I’m pretty sure that even with a default camera, if you add a sphere to your scene and move it to the corner of your frame, you get some visible distortion. I had changed some of the camera settings, though. It’s actually an old project that I’m revisiting and converting to 16:9, and the distortion was more evident in the wider aspect ratio.