Optics any opening in an optical system that allows light to pass.
Pupils are the beam-limiting apertures of a system.
Optics softening the pupil edge to reduce or eliminate ringing of the diffraction spot. Ophthalmology physical aperture stop of the eye formed by the iris and controlling the amount of light entering the eye. The entrance pupil is the image of the physical pupil generated by the cornea. The apparent pupil seen by external examination is imaged through the cornea and is therefore an entrance pupil. The exit pupil is the image of the physical pupil generated by the crystalline lens.