Pupil: Anatomy, Function, and Related Eye Conditions

Published on March 1st, 2025

The pupil is the black circle in the middle of the eye, surrounded by the iris and covered by the cornea. The pupils are the parts of the eye that constrict and dilate in response to light, controlled by the iris. Pupil-related eye conditions include anisocoria, cataracts, miosis and Horner syndrome. Different ways to take care of the pupil include booking regular eye tests, stopping smoking and eating a balanced diet. Keep reading to learn more about the anatomy of the pupil and how it helps the human eye see.

What is the Pupil?

The pupil is an opening in the iris located directly in the centre of the eye and covered by the cornea. Light passes through to the retina as the iris adjusts the size of the pupil, controlling the amount of light that enters the eye, according to the National Eye Institute. The pupil is surrounded by the iris, which is the coloured part of the eye.

What is the Colour of the Pupil?

The pupil is black in colour as it absorbs all the light that goes into the eye. No light is reflected back from the pupil, therefore the pupil is black. The coloured part of the eye is called the iris and surrounds the pupil.

Does the Colour of the Pupil Change?

The colour of the pupil generally does not change although certain eye conditions can make the pupil look different. A cataract on the lens can make the pupil appear white or cloudy. A cataract is a clumping of protein in the eye’s lens caused by age-related changes in the eye.

Where is the Pupil Located Inside the Eye?

The pupil is a round opening in the iris located directly in the centre of the eye and covered by the cornea. The iris is the coloured part of the eye and controls the size of the pupil in response to light.

A diagram of where the pupil is located in the eyeball.

What is the Function of the Pupil in the Eye?

The main function of the pupil is to let light into the eye so it can be focused in the retina by the eye’s lens and transferred into images. The iris controls the size of the pupil to maintain how much light is allowed inside of the eye depending on the volume of the light being viewed. The second function of the pupil is to help the aqueous humour. The aqueous humour moves through the pupil to the front of the eye which then drains out of the eye into a person’s bloodstream, according to the Cleveland Clinic.

How Does the Pupil Help the Human Eye See?

The pupil helps the human eye see by allowing light into the eye so it may be focused on the retina by the eye’s lens and transferred into images. This occurs as the iris dilates or constricts the pupil to control how much light enters the eye. Once light enters the eye, it is directed onto the retina which is then converted into electrical impulses. The optic nerve then carries these electrical signals to the brain where finally the brain turns these signals into images. Many parts of the eye are involved in how humans see.

Is the Pupil Sensitive to Light?

The retina is the light-sensitive part of the eye, according to the National Eye Institute, not the pupil. The retina is located at the back of the eye and converts light that enters the eye into electrical impulses which the optic nerve then sends to the brain.

What are the Pupil-Related Eye Problems?

The pupil-related eye problems include anisocoria, cataracts, miosis, Horner syndrome, polycoria, and Adie syndrome. These eye conditions and their definitions are listed below.

Anisocoria: Anisocoria is a condition where the pupils are uneven in size. Anisocoria may only be temporary or can be caused by serious conditions. This eye condition may cause vision issues and a sensitivity to light.
Cataracts: A cataract is a clumping of protein in the eye’s lens which causes the lens to become cloudy instead of clear, called opacification. Cataracts are an age-related eye condition that causes sensitivity to light, double vision and blurred or cloudy vision.
Miosis: Miosis, also known as pupillary miosis, is the medical term for very small or constricted pupils, according to the Cleveland Clinic. This eye condition causes the pupils to stay small even when it’s dark.
Horner syndrome: Horner’s syndrome is a rare eye condition caused by damage to part of the sympathetic nervous system. Common symptoms of Horner syndrome include a small pupil, partially drooping upper eyelid and loss of sweating on the face.
Polycoria: Polycoria is a rare eye condition where a person is born with multiple pupils in one eye. There are two types of polycoria, true and false polycoria. Symptoms of this eye condition include blurry vision, glare sensitivity and double vision.
Adie syndrome: Adie syndrome is a common neurological condition comprising bilateral or unilateral tonically dilated pupils, according to the National Library of Medicine. The cause of this eye condition in most people is unknown but it can occur due to trauma or infection.

An eyeball that has a cataract on the lens which affects a person’s pupil.

What is the Role of an Optometrist in Diagnosing Pupil-Related Conditions?

An optometrist plays a vital role in diagnosing conditions related to the pupil. An optometrist performs many tests in a comprehensive eye test to detect any eye conditions and assess a patient’s vision. An optometrist may also refer patients with more complex diseases to an ophthalmologist for further diagnosis and treatment. For example, while cataracts can be diagnosed by an optometrist, if the patient requires cataract surgery, this will be performed by an ophthalmologist.

Is the Pupil the Most Commonly Damaged Part of the Eye?

No, the pupil is not the most commonly damaged part of the eye. The pupil cannot be directly damaged, however, damage can occur to the iris or the nerves connected to the pupil. A pupil that is largely dilated or non-functioning, known as a blown pupil, is very serious as it may be caused by severe brain trauma such as a stroke.

What are the Different Ways to Take Care of the Pupil?

The different ways to take care of your eye health, including the pupil, is to book regular eye tests, stop smoking and eat a balanced diet. These actions and their definitions are listed below.

Book regular eye tests: Booking regular eye tests is very important in taking care of your overall eye health including the health of your pupils. Eye tests allow optometrists to monitor the health of your eye and detect eye diseases.
Stop smoking: Smoking is very detrimental to a person’s eye health as it has been shown to cause damage to the retina and the macula. Smoking is also linked to an increased risk of age-related macular degeneration.
Eat a balanced diet: A healthy and balanced diet is beneficial for overall eye health including the pupil. By eating well, you give your body the fuel it needs which helps to boost immunity and health.

What is the Importance of a Regular Eye Test for the Pupil?

An optometrist can perform an eye test to check the health of the eyes including the pupil. In a regular eye test, an optometrist will test the function of the patient’s pupils, among other tests for eye health and vision. A pupil function test is used to assess your pupils' physical condition, which may involve shining a light on each of your pupils to test if they constrict in response to light and will record the size and shape while this takes place. This assists the optometrist with evaluating the health of your retina, optic nerve and brain.

How can Oscar Wylee Help Take Care of Your Pupil?

Oscar Wylee can help you take care of your eye health, including your pupils, through regular eye tests. We offer bulk billed eye tests for valid Medicare cardholders, performed by trusted and skilled optometrists with high-quality equipment. We recommend everyone should have an eye test at least once every two years, and once a year for people over the age of 65.

Why are Eye Drops Used on the Pupil?

Special eye drops are used in an eye test to dilate a person’s pupils. These drops allow the optometrist to shine a light on the pupil without them getting smaller. Dilating eye drops are used so an optometrist can look at the deeper structures of the eye and will make a person’s vision blurry for some time after use.

A person having dilating eye drops put in their eyes.

Can the Pupil Repair Itself?

No, there is little evidence to suggest the pupil can repair itself. Damage to the pupil is not directly possible, however, damage to the iris or the nerves connected to the pupil can result in light sensitivity during the day, or when looking at bright light and glares.

What is the Difference Between the Pupil and the Iris?

The difference between the pupil and the iris is their appearance and anatomy. The pupil is the black circle in the middle of the iris. The iris, which is the coloured part of the eye surrounding the pupil, controls the pupil by dilating or constraining it to let light in.

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