Although the pupil is not a refractive element in the eyeball optical system, it can not only respond to light and shade, adjust the light entering the eye, but also affect the focal depth and spherical aberration of the eyeball optical system.

Pupils can vary widely. In extreme contraction, the diameter of human pupil can be less than 1mm, and in extreme expansion, it can be more than 9mm, and the iris sphincter can be contracted to 87% of its length.

This is almost impossible for other smooth muscles or striated muscles of the human body to achieve. The pupil itself will adjust its size according to the intensity of external light. When the light is strong, the pupil will become smaller, and when the light is weak, the pupil will become larger. This is self-protection for the eyes.