What is the function of the iris of the eye?

Updated April 17, 2017

The eye is our most important optical instrument, giving the sense of sight and adding colour and perspective to our vision. It is a very complex organ and is made up of five intricate parts. One of the most important parts is the iris.

Parts of the Eye

There are five important parts of the eye; the cornea, pupil, lens, retina and iris. All of these parts need to work correctly for clear vision. The eye also has a blind spot, which is the point where the optic nerve enters the eye, rendering the area insensitive to light.

Location and Composition

The iris is in the anterior compartment at the front of the eye. It lies between the cornea and the crystalline lens. When looking at your eye, it appears as the coloured disk but it is actually a thin diaphragm that is made up of a collection of muscle fibres and connective tissue. The iris is made up of three layers, which, from front to back, are the endothelium, the stroma, and the epithelium.


The iris acts as a divider between the large anterior chamber, which is between the iris and the cornea, and the small posterior chamber, located between the iris and the lens.

The eye can be compared to a camera. In this analogy, the iris and the pupil act like the aperture of the camera, with the iris functioning like the camera's shutter. At night, when less light enters the eye, the iris dilator muscle pulls outward from the centre. This causes the pupil to get bigger, or dilate, allowing more light to enter the eye. Conversely, when there is a bright light, the iris sphincter muscle causes the pupil to become smaller (constrict) by pulling toward the centre. This allows less light to enter the eye, preventing damage of the retina.

The iris also can be used to identify people, in a similar manner to a fingerprint, because the colour, texture and pattern of every iris is unique.


The iris and its surrounding components, such as the retina or uvea, can become infected or diseased. Common problems include iritis, which is when the iris becomes inflamed and causes light sensitivity and blurred vision, as well as iridocyclitis, which affects the ciliary body and the iris. Both of these can be treated with antibiotics, but it is important to catch them early because if they are not treated, they can lead to a permanent reduction in vision and even blindness.

Color of the Iris

The amount of pigment in the iris determines its colour. This is established genetically. For example, if there is no pigment, the eye will be pink or red, whereas a lot of pigment will cause the iris to be hazel or brown.

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