How does the pupil of our eyes help us to see?
The pupil is a central opening in the iris of the eye, The iris is a muscle sphincter that dilates or constricts in reaction to things like brightness, glare, shifting one's attention to close-up reading distance, etc. The smaller the pupil, the less that small distortions in the eye can affect vision (e.g. astigmatism). The larger the pupil, the more light scatter can occur causing veiling glare and the shorter the depth of focus.
_Written by J. Trevor Woodhams, M.D. - Chief of Surgery, Woodhams Eye Clinic