During the recent SpaceX CRS-12 launch which was broadcast live on YouTube, it appeared at one point as if the horizon of the earth was completely flat.
What caused this to happen?
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I suspect it is a combination of the fisheye effect of the camera lens and the lack of altitude.
The fisheye effect is well known for making the earth appear more round than it should be. It does so by curving seemingly straight lines towards the center of the image, with greater effect the further from the center. Take the opposing camera angle from the same launch:
Here, the fisheye effect makes the earth more curved than usual as the curvature of the earth is added to the curvature due to the wide-angle lensing. Since in your screenshots, the earth is in the corner away from the centerpoint, the fisheye effect tries to reverse what curvature there was away from the center, the net effect making the earth flat.
Also, at an altitude of ~200km, the curvature of the earth is not very apparent. Even with the naked eye, you would only notice the curvature due to your wide angle of vision.