Why Our Eyes Have Pupils

The hole or opening that is located in the center of the iris of the eye. The pupil controls the amount of light that enters the eye. Pupil size is controlled by the dilator and sphincter muscles of the iris.

Extreme Close-Up Of Human Eye
Getty Images / Chris Raven / EyeEm

Why We Have Pupils

The pupil controls how much light is let into the eye. It is very similar to a camera aperture which allows more light in for more exposure. At night, our pupils dilate to allow more light in to maximize our vision. In the bright sunlight, our pupil shrinks to a very small diameter to allow us to function normally. Otherwise, we would be very light sensitive, and this could effect photoreceptors in our retina.

Also, when we look at something at a very close distance such as reading a book, our eyes converge and our pupils shrink. When our pupils shrink, it is similar to looking through a pinhole. Looking through a small hole reduces peripheral blur and increases the depth of focus. This improves overall visual acuity. Normal pupil size is between 2.5 to 4.0 mm.

What System Controls the Pupil?

The iris, the colored part of our eye, is made up of pigment and contains two sets of smooth muscles that control the size of the pupil: the sphincter muscle and the dilator muscle. The sphincter muscle is in the shape of a ring at the margin of the pupil. When it contracts, it constricts or decreases the size of the pupil. The dilator muscles are in a radial shape throughout the iris and when it contracts, it dilates or increases the size of the pupil.

Both systems, the parasympathetic and sympathetic systems control the pupil. Our parasympathetic system controls everyday activities such as rest, slowing the heart rate and things like digestion. It controls the pupil size during normal activities during the day and acts to change the pupil size depending on how much light is present. The sympathetic system is a protective system and gives us the typical "fight or flight" responses. In the pupil, if we are scared or feel fear, our pupils dilate very large. This is thought to allow light in so our responses are quicker. 

Significance in Healthcare

Examination of the pupil is important because it can denote problems in the pathways controlling the pupil. A light is used to check the pupils, first to check the size and symmetry on each side, and then the light is swung from side to side; A "relative affarent pupillary defect" occurs when the intensity of light entering the eye is not transmitted along the light reflex pathway, and instead of the pupil becoming smaller with the bright shiny light, it becomes bigger. It is a paradoxical movement suggesting that there is a problem along the visual pathway of the affected eye.

When pupils are uneven, it is called "anisocoria." If one of your pupils appears smaller than the other and this difference is more exaggerated in the dark, it could mean that there is a problem in the pathway that controls dilation (the sympathetic chain of nerves). This sympathetic nervous chain starts in the brain, goes down the spinal cord, courses along the top of the lung, and goes back up into the neck along the carotid artery in the neck. A small "constricted" pupil could result from trauma, problems with large neck blood vessels, or tumors at the top of the lung, interfering with the signal of this nerve pathway. A problem along the parasympathetic pathway means that the pupil has trouble staying constricted and results in a dilated or "blown pupil." In rare cases, a brain aneurysm can cause this. Therefore, any sudden changes in pupil size should be examined immediately by an ophthalmologist.

Frequently Asked Questions

  • What causes uneven pupils?

    Uneven pupils can be caused by brain aneurism, blood vessels in the neck being torn or blocked, and as a reaction to certain topical dilating medication that only reaches one eye. However, people born with uneven pupils and experience no harmful symptoms shouldn't need to worry. This is known as physiologic anisocoria and isn't cause for concern.

  • What is the lens?

    The lens is a transparent eye structure that helps us focus on things at varying distances. It is located behind the pupil and iris. The natural lens that we are born with is called the crystalline lens.

1 Source
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Johns Hopkins Medicine. Pupillary Disorders Including Anisocoria.

By Troy Bedinghaus, OD
Troy L. Bedinghaus, OD, board-certified optometric physician, owns Lakewood Family Eye Care in Florida. He is an active member of the American Optometric Association.