Circular aperture in optical instruments Why do almost all the optical instruments have a circular aperture to gather light? For example, objective lens of telescope, shape of pupil, lens of camera and so on. Can anyone please explain me relating it to resolving power?
 A: There's plenty of telescopes that use non-circular primary mirrors, particularly when they are composite mirrors made up of multiple individual components:

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Circular primaries, along with lenses and other optical elements, tend to be easier to manufacture and to polish, because if the target shape is symmetrical then you can use a symmetrical construction / polishing method (see e.g. this video for an example in action). However, once you get to a certain size, it becomes impossible to build single-piece primary mirrors, and you need to move to a segmented primary (which also handles adaptive optics better), in which case the preferred shape is a collection of hexagons, which provide the best tesselation of the plane.
For your run-of-the-mill camera lens, if the element is symmetric then that allows for a symmetric shaping, which helps avoid astigmatism - an otherwise generic type of aberration that occurs in cylindrically-asymmetric systems. You can polish a square lens so that it is astigmatism-free, but it's more work than for a circular one.
And as for camera apertures, while some shutters try to approximate a circular aperture by a polygon with lots of sides (example), there's plenty of cameras out there with straight shutters that travel linearly:

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A: Lenses could be made rectangular, or nearly any other shape, but they would prove to be less practical and harder to manufacture than circular ones. 
For example, the sensor in a camera is rectangular, but that's only because we are used to the convention of rectangular pictures in stead of round ones.
