aperture stop

In optics, an aperture stop is the physical element within an optical system that limits the diameter of the light beam passing through it. This stop determines the amount of light that enters the system, directly influencing the brightness of the resulting image. The size of the aperture stop is often dictated by the mount holding an optical component or a diaphragm. The image of the aperture stop, known as the entrance pupil, is crucial for measuring the light-gathering power of an instrument. In photography, aperture settings, expressed as f-stops, control the amount of light reaching the film.123