I'm using SceneKit. I have created and assigned my own camera to the scene and I have adjusted its xFov and yFov. When I set a value higher than 50, there starts to be some distortion. Everything near the edges of the screen is stretched – almost like the camera suddenly becomes a "Fish Eye."
I need the xFov and yFov to be above 50 (I actually need it to be 100), but I can't have that distortion. What do I do?
What you're asking isn't theoretically impossible per se, but theoretically interesting at least.
What happens to a physical camera when you increase the field of view? The wider it gets, the more "fisheye" it looks. The projection matrix and perspective divide of a 3D graphics pipeline like SceneKit works in a similar way. It looks a little different because it's a rectilinear transformation rather than the effect of a spherical lens, but it's the same general idea — it maps a volume (called a frustum) of 3D space "seen" by the camera onto the viewing plane. This is a general aspect of 3D graphics, not something specific to SceneKit, so you can find plenty of good tutorials that cover the underlying math pretty well.
That frustum projection fixes a certain relationship between the amount of viewing angle something takes up and its width on the viewing plane. You can't really change that relationship and still have a linear (well, rational, but mostly linear) transformation that 3D hardware can apply with a single matrix multiplication (and perspective divide).
You could, in theory, define a different relationship — say, one where a large angular size corresponds to a much larger part of the viewing plane near the center of the view, but to a much smaller part farther away from the center. But you can't do that in the camera transform... You'd have to do such calculations pixel by pixel in some kind of post-processing shader. (In fact, this is generally how rendering for the lenses of a VR headset works.)