A good majority of pictures of Earth taken from space are being dismissed by flat earthers simply because they are taken using fish-eye lenses. They believe any curvature of Earth shown in these picture are caused by fish-eye effect, and thus, cannot be used as evidence of Earth curvature. They even take a step further: any picture showing curvature of Earth must have been taken by a fish-eye lens.
These claims are, of course, ungrounded. In some cases, we can conclude the curvature is there, even when a fish-eye lens is being used. We can use this useful property of any photographic lens:
A straight line will appear straight as long as it crosses the center point of the image.
We can use this property in order to determine whether there is a curvature or not. Doesn’t matter what distortion your lens has, the property is always applicable: minor distortion on rectilinar lenses, barrel distortion on fish-eye lenses, pincushion distortion on telephoto lenses, wavy/mustache distortion on some zoom lenses, you name it.
If the horizon line crosses the center point of the picture, we can conclude whether the horizon is straight or curved. If the horizon appears to be curved, then it must be curved in reality.
In particular, videos are really easy. In any video of the Earth taken from space, there should be a lot of moments where the horizon line crosses the center point of the picture. We can simply find one of these moments and pause on it. If it is curved, then it must be curved in reality.
There’s an exception though: the picture can’t be cropped off-center. But it should be easy to spot off-center cropped pictures: the distortion will no longer have symmetry. Center-cropped pictures are OK.