It seems you guys don't grasp it, do yah?
There is no horizontal curvature on the circled horizon around you, and I am talking about oblate spherical planet.
If it exist, so it would accumulate and go very deep down on your back view, right?
The problem on any optical device is the lens, only expensive lens can give you a very good orthoscopy image.
You need to read https://clickitupanotch.com/lens-distortion/
There is no horizon curvature, except if you go very very high in a way where you have the whole object in front of you, nothing on your back. In case of Earth planet, "very high" means more than 20 thousands miles up.
Apparently this thread is being diverted to discuss camera pixels accuracy, trying to ignore the original post.
I believe you are mistaken. Yes there IS a horizon curve. I just posted a video showing you that it's there. I also explained how any amount of lens distortion affects the straight-edge in exactly the same way as the horizon. You can then compare the curve of the straight-edge to the curve of the horizon to see what the true curve of the horizon is. It's curved, and if you'd care to do the math, it matches amazingly well.
Am I misunderstanding you here? Let me try to lay out some logical flow in case we're not communicating effectively.
1) Imagine that the horizon is a circle equally distant in all directions.
2) That circle is level. It is perpendicular to the direction of the pull of gravity.
3) You are in the center of that circle.
4) That circle is below eye-level very slightly.
Right? We're all on the same page so far?
5) Let's take that "below eye-level" to the extreme and explore what that would cause. Use the hula hoop visualization. Stand in the middle of a hula hoop and take a photo of the front of it. The front of the hoop is dead center in your frame. To the right, the hula hoop exits the frame lower than that. To the left, the hula hoop exits the frame lower as well. You're looking down on a circle, so of course it looks curved.
6) Now slowly raise that hula hoop up towards eye level. At what point does that curve finally become a straight line?
7) It's a straight line when it's at eye level, and it's a curve on the floor. How does it go from one to the other?
There won't be any discontinuity. It's going to smoothly transition becoming less and less curved as you raise it until it finally hits perfectly straight at exactly eye level.
Right?