Why Does Your Pupil Change Size?

by Jan 18, 2023

The pupil is the black part of the eye that is surrounded by the colored iris. The pupil is an opening that allows light to pass through the iris and into the back of the eye. Depending on the amount of light in the environment, the pupil can be of different sizes. The iris is responsible for changing its shape to adjust how large the pupil is. Pupil size is important to maximize vision without causing unwanted aberrations.

The Pupil and the Iris

The eye is designed to be able to adjust pupil size easily by constricting or relaxing the iris.

The iris will act as a shutter of a camera that can shrink the opening of the pupil when there is ample light around and then increase the opening when there is less light.

The iris is controlled by the autonomic nervous system, meaning there is no conscious effort put into changing the pupil size, instead it is regulated automatically just like the heart rate.

In some individuals, the pupil is normally slightly larger than in others. This is a normal part of variation among individuals, but the pupil will still constrict in bright light and dilate in dim light.

Benefits of a Small Pupil

A small pupil is beneficial in bright light conditions because it will limit the amount of light that enters the eye.

In bright conditions, the eyes can actually receive too much light from the environment, and it can cause the vision to be washed out or seem blurred.

A perfect example of when the pupil is too large for the environment is when leaving a dark movie theater on a sunny day, the light from the outside will be overwhelming and seem unbearably bright.

However, after just a few seconds to a minute, the light is no different than what is normal for a sunny day.

This happens because the pupils are dilated from the long exposure to the dark movie theater and are quickly switched to the bright environment.

Benefits of a Large Pupil

Conversely, a large pupil is beneficial for allowing as much light as possible to enter the eye to help create a usable visual image.

In the dark, even a small amount of light can be used to help create vision.

By increasing pupil size, the eye is naturally able to increase the ability to capture light and thus improve vision.

Small lights or lights that are in the peripheral vision can be used with a large pupil to help piece together the entire visual field when the pupils are large.

A common issue with the natural dilating of the eyes is when driving at night.

At night, the environment is usually dim enough to cause the eyes to dilate and increase pupil size, however, when oncoming headlights are shone directly into the eyes, the dilated pupil size is not advantageous.

This can cause blurry vision, glare, or halos at night and is one of the biggest complaints associated with driving at night.


Our eye doctors at Perception Eyecare + Eyewear in Richardson, TX excel in the prescription of contact lenses, glasses and various eye diseases.  Call our optometrists at 972-250-0700 or schedule an appointment online if you would like to learn more about the pupil and why it changes sizes. Our optometrists provide the highest quality optometry services and eye exams in Richardson, TX and the surrounding vicinities of Garland, University Park, Sachse, and Highland Park.

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