eli5 Why are camera lenses round when all media we view is square or rectangle
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The lens is round because it’s easier to manufacture round lenses than square ones. The media is rectangular because film comes in strips and rectangles are a more efficient use of filmstock.
Electronic sensors and chips are easy to make rectangular. In particular, a cmos or digital camera sensor is just an x by y (your resolution) grid of little light detectors. You can make em round, but you’re just wasting pixels and board/chip space.
Meanwhile lenses are easy to make round and super hard to make square – actually they just make a bigger round one and grind it into a square shape. But essentially whats going on in your cell phone is the lens is right shaped and sized to transmit and focus light into a circle into which our rectangular sensor exactly fits. So technically we’re wasting a bit of the light that comes through that lens. Im not an optics guy but Im willing to bet if you made a rectangular lens that exactly covered just the sensor, you’d have a lot of weird distortion effects in the corner you’d have to compensate for, depending on how you centered the center of the original round lens when you ground it rectangular. And you tend to get more distortion effects towards the edge of any lens anyway**, so if you just use a larger lens, the distorted bits at the edge are just ignored because you have no sensor there.
**at least in the lenses that my company’s product use anyways.
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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