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.
A straight line will appear straight if the line crosses the image’s center point. This characteristic is helpful if we want to determine if a line is straight, even if a fisheye lens is being used.
This characteristic applies to any distortion with any photographic lens, not just fisheye or barrel distortion. A telephoto lens usually has a slight pincushion distortion. A consumer-grade zoom lens usually has a mustache/wavy distortion.
If the line that crosses the center point appears straight, it is straight in reality, irrespective of the lens being used. Conversely, if the line that crosses the center point appears curved, we can say it is also curved in reality.
The characteristic is more useful for videos showing the horizon. There will be many scenes where the horizon line crosses the center point. We can pause at such a moment and observe the result. If the horizon is curved, then it is also curved in reality.
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