When will a pupil dilate or constrict?
The pupil of the eye is an aperture (hole) in the iris that limits a light-bearing image coming through on its way to being focused on the retina. It typically bounces back and forth between a slight constriction and slight dilation (called "hippus").
The iris (the colored part of the eye) is actually a muscle sphincter that has constricting fibers and dilating fibers. The dilating fibers are stimulated by decreased light, but also by positive emotional reactions! [Some poker players and buyers at auctions wear sunglasses so that the person on the other side of the table cannot "read" them. That other person might know their opponent likes the cards or product he sees by the dilation of their pupils.]
The constricting fibers of the iris are stimulated by reaction to increasing light, …but also by shifting focus to reading distance from far distance.
So the dilation or constriction of the pupil is primarily a response to light conditions, but also at times due to other stimuli.
_Written by J. Trevor Woodhams, M.D. - Chief of Surgery, Woodhams Eye Clinic