What is an Eye Pupil?

Diagram showing the iris and pupil of an eye

Pupil Defined

The “pupil” of the eye is the hole created by the iris (the color part of the eye) through which light is allowed to travel from the front of the eye to the back of the eye.

The term “pupil” stems from the Latin word “pupilla” which originally meant, “little girl-doll”. The Greeks had a word (“kore”) that meant both “doll” and “the pupil of an eye.” Eventually, the Latin term for “little girl-doll” (pupilla) was translated into English to the word “pupil”, but carried the Greek’s meaning of “the pupil of an eye” along with it.

The root words “pupill-” and “core-” are often used when medically defining conditions that are associated with the pupil. A few examples are “Pupillomoter” which is a device that measures the distance between pupils, and “Anisocoria” which indicates different sized pupils.

Scientific Studies Involving the Pupil

Optometry and Ophthalmology are both practices that scientifically evaluate the pupil and iris along with ALL of the structures that constitute the eye. The pupil and iris together often direct attention to certain diseases of the body and brain, and therefore are important structures to be evaluated within an eye exam.

Iridology, however, is a non-scientific interpretation of the structures and shapes of the iris and pupil that is sometimes utilized in homeopathic care. Be aware, iridology is not science based and may rightly be considered in line with palmistry (the study of lines on a person’s lines of their hand, “palm reading”) and phrenology (the study of a person’s bumps on their head) in determining health issues. Again, it is unrecognized scientifically and should only be used for its entertainment value and not as a diagnostic tool for health. Though people who practice iridology may have good intentions, and altruistically want to help others, the teachings of iridology are misleading. Also, you are likely to find an iridology exam followed by the selling of vitamins or other treatment suggestions.

The Pupil’s Role in Eyesight

Have you ever seen a color photograph that used a flash bulb that resulted in a red dot in the center of the pupil. This red dot is a reflection off the retina from the back of the eye. The pupil is a hole for light to enter the eye and be absorbed by the retina, but in that precise moment of the camera’s flash, some of the light is reflected off the reddish-orange colored retina and comes back through the pupil to be captured in the photograph. The red reflection demonstrates that the pupil is nothing more than a hole (but a hole with a purpose).

The pupil is the only hole through which light enters the eye to eventually be focused in the back of the eye at a point called the “fovea”, which is the most important area of the “macula”, which is the area for all central vision of the tissue called the “retina”, which is a membrane that is connected to cranial nerve 2 (the optic nerve) through which received retinal images are sent to the brain for interpretation.

The pupil can alter its size to modify the amount of light that enters, and is often compared to the “aperture” on a camera. In low lighting conditions, the pupil gets larger (dilates) to allow more light to enter the eye. In brighter light conditions, the pupil gets smaller (constricts) to reduce the amount of light that enters the eye. Signals from Cranial nerve 3 (the oculomotor nerve) control pupil size.  

Pupils also get smaller when focusing (as with reading a book), and get larger when relaxing to look further away. Since people who are farsighted have to focus (even when looking far away), their pupils tend to be very small when they are not wearing their glasses. The opposite can be true as well. A nearsighted person relaxes their eyes to see without their glasses when they are looking far away, and tend to have larger pupils.

As we age, our pupils naturally reduce in size.

How do Contact Lenses Work with the Pupil?

Since light enters the eye through the pupil, the only part of a contact lens that requires corrective power is the center area directly over the pupil, called the Optic Zone of the contact lens. Since the pupil can change size, the Optic Zone is usually made to be larger than a pupil at its largest size.

Multifocal soft contact lenses require more than one power in the Optic Zone. Both far and near powers of the lens must be aligned within the Optic Zone of the contact lens.  The multifocal contact lens design takes into account the fact that the pupil gets smaller when the eye focuses at near, so the near power of the contact lens is most commonly centered in the middle of the optic zone, while the distance correction usually surrounds the near zone and covers the outer ring of the Optic Zone.

Common Vision Conditions that Involve the Pupil

Remember: the pupils are controlled by the brain, and the pupils react together. Therefore, if you shine light in one pupil, it will get smaller, and so will the other pupil that did not have a light shined into it. If the pupils do not react together, that is a bad sign and a doctor should evaluate immediately. Actually, anything that affects the brain can affect the pupil size. Here things that can make the pupil bigger (dilate) or smaller (constrict).

Bigger Pupils (dilated):

  • Idiopathic (normal for that individual)
  • Low light
  • Myopia (nearsighted)
  • Youth
  • Distant viewing
  • Head injury
  • Brain problem
  • Eye disease
  • Systemic conditions
  • Drugs

Smaller Pupils (constricted):

  • Idiopathic (normal for that individual)
  • Bright light
  • Hyperopia (farsighted)
  • Older age
  • Near viewing
  • Head injury
  • Brain problem
  • Eye disease
  • Systemic conditions
  • Drugs

Tips for Maintaining Eye Health of the Pupil

Since the pupil is a hole formed by the iris which is controlled by the brain, anything that you can do to maintain good neuronal health can be beneficial to pupil health. 

For example (but not limited to): cardiovascular health, proper nutrition, and mental stability. 

Learn about more parts of the eye! →

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