I'm on the train going home from work and I can seet the feet of the people standing across from me.
If you can see their feet while looking straight ahead, they are within your eye level.
But it is likely you are using your peripheral vision to see the people's feet.
In your image one can see the full little black outline, and horizon by simply looking straight without using peripherals.
Extending out from your eyes at an angle of 0° (zero degrees, horizontal from the eyes)
In other words you can look straight ahead and see it. All of the photos you show have the horizon at your defined eye level.
Because an angle of other than 0 means one would have to be moving their eyes or using their peripheral vision.
If the eyes can remain exactly straight and see the thing, it is at eye level.
A 0 angle eye is an eye that is looking perfectly straight ahead and not using peripherals.
No, on all but the first image at near sea level you look down slightly at the horizon. If the earth was flat then the horizon would always be exactly at eye level due to perspective, no matter what altitude you are viewing from.
You do not look slightly down, don't even have to use peripherals, the horizon is straight in front at 0 angle eye level.
You can also see above the horizon without having to move the eye, the whole horizon area is at 0 angle eye level,
in every photo with the red water.
Not if the earth is flat. From high up you can see much further and the horizon would again be at eye level.
That's right, as it is in the photo you gave with the water bottle.
As I said, a land shot would show it better, the horizon is within eye level.
If the earth were a ball, from higher and higher up one would have to look more and more down to see any horizon.
So on a plane would should have to look down, but as in your photo we can look straight ahead and see the horizon.
It would be a better example if it were land and not clouds.