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In optics, the word aperture describes a hole or opening through which light travels. Aperture is crucial in a number of optical instruments such as cameras and telescopes. Adjustable apertures, manual or automated, are typically used in applications where limiting or shaping incident light is required. For example, rectangular apertures can be used to define the boundaries of an optical path or to mask out specific areas in an optical system, like on a test target, monochromator, CCD or a detector.