eli5 Why are camera lenses round when all media we view is square or rectangle
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ELI university education: The shape of the lens/aperture is not the same as the shape of the image. An aperture actually produces an image that is the Fourier transform of the shape of the aperture. We use a circle because the Fourier transform of a circle happens to also be a circle, and the mathematics of forming a useful image with that are relatively easy. If we wanted a rectangular image, the aperture would need to be shaped like the Fourier transform of a rectangle, which is a super complicated shape with lots of spiky bits. Similarly, if the aperture was rectangular, the image shape would *not* be a rectangle – it’d be the Fourier transform of a rectangle – an irregular and very spiky shape. You’d be blocking a lot of the important spatial frequencies that go into making an acceptable image. In other words, the image quality would be *awful*.
ELI5: A circular lens produces a nice circular image, which we can very easily crop to make a rectangle. But a rectangular lens *doesn’t* produce a rectangular image (because complicated physics/mathematics) – it actually makes a huge mess and the resulting image would be worse quality. So we don’t do that.
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