In an optical system, Apertures that limit the lens or optical system devices, such as plates with holes, or the apertures or frames that limit the imaging range, such as aperture shutters, are collectively referred to as "apertures". A diaphragm is an optical element in an optical instrument (optical train).

According to their different functions, it is divided into two types: aperture diaphragm and field diaphragm.

The aperture stop is a hole that limits the size of the incident beam, and its size and position have a decisive effect on the clarity, correctness and brightness of the image formed by the lens. Such as the circular diaphragm (commonly known as the aperture) on the camera lens.

The field diaphragm is an aperture used to limit the size of the area (field of view) of the imaged scene. For example, film frames in photographic systems.