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aperture ap·er·ture (āp'ər-chər)
An opening, such as a hole, gap, or slit.
A usually adjustable opening in an optical instrument, such as a microscope, a camera, or a telescope, that limits the amount of light passing through a lens or onto a mirror.
The diameter of such an opening.
The diameter of the objective of a telescope or microscope.
in optics, the maximum diameter of a light beam that can pass through an optical system. The size of an aperture is limited by the size of the mount holding the optical component, or the size of the diaphragm placed in the bundle of light rays. The hole in the mount or diaphragm that limits the size of the aperture is called an aperture stop. Thus, an aperture stop determines the amount of light that traverses an optical system and hence determines the image illumination