A photographic lens may have a specific distortion characteristic and strength that may differ from another lens. However, despite the differences, a straight line will always appear straight if it crosses the center of the frame.
Flat-Earthers like to dismiss images taken by a fisheye lens. In reality, the attributes of such a lens are well known. We can use the knowledge to determine if a line in the image is straight, even if it was taken using a fisheye lens.