Why Fisheye Lenses Are Used

Cameras with a fisheye lens are used in the ISS, high-altitude balloons, rocket bodies, and other purposes. The reason is that fisheye lenses have several advantages over rectilinear lenses.

Flat-Earthers claim there is an evil intention behind the usage of fisheye lenses. In reality, the use of fisheye lenses is not unreasonable. If it is desired to have a rectilinear image output, then it is not hard to defish fisheye images to rectilinear ones..

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Infinity Focus

To an optical device, like a camera, there is little difference between an airplane at 20000 ft and the Moon at 384400 km (238855 mi). The aperture of the lens is far too small compared to the distance of either object. Focusing on either object will make the other object in focus, too.

Flat-Earthers claim that because both objects appear in focus in a photograph, then the Moon must be close to the airplane. In reality, both can be considered at infinity. And it will be easy to make both objects to be in focus simultaneously.

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