Some context. Above are the code and the resulting image for it in Peter Shirley's Raytracing in one weekend's book. As you can see from the code, he adds in some spheres. And yet in the final image there are 2 ellipsoids side by side. I just can't fathom why it is so, no matter how many time I looked at the code. Can anyone explain to me? Here's the link to the source code. https://github.com/petershirley/raytracinginoneweekend
Shapes appearing stretched in the periphery is a consequence of perspective projection. The wider the field of view (FOV) is, the stronger the stretching effect gets.
To demonstrate the effect I wrote a quick example on ShaderToy: https://www.shadertoy.com/view/MltBW2
As you can see on the images below (corresponding to FOV of 40, 80 and 120; if I didn't make any mistake), this becomes especially noticeable with wide FOV, like 90° and above.