I'd just like to point out that the true curvature of the Earth is visible in all the weather balloon videos out there on YouTube. It is true that the camera lens is distorting the horizon across the screen from left to right, but that isn't the only curving of the Earth in these pictures. In all the videos you can see that the Earth curves away from us into the distance. If the Earth was flat, we should be able to see hundreds or thousands of miles away into the distance as we rise higher. Instead we always see the edge of the Earth in a sharp line as it curves away from us.

This screen shot at 6:16 from the video posted above shows the horizon from left to right looking as if the Earth's curve is concave. We all know that is not correct, it is a lens distortion. However in this same image, we can see that the Earth curves away from us into the distance. It is very clear that anything moving away from you would fly out and down around the bend and out of sight as the Earth curves away. Every balloon video that I've ever watched on YouTube shows this same thing; even the ones claiming 100% proof that the Earth is flat. As I look at them, they all show camera lenses distorting a picture of a spherical Earth curving away out of sight.