Aperture

Hole or opening through which light travels

In optics, the aperture of an optical system (including a system consisting of a single lens) is the hole or opening that primarily limits light propagated through the system. The aperture defines a bundle of rays from each point on an object that will come to a focus in the image plane. An optical system typically has many structures that limit ray bundles (ray bundles are also known as pencils of light).

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