Iris & Pupil

The iris is a curtain separating the front (anterior) chamber from the back.

It has a hole in the middle called the pupil.

The iris cuts down the amount of light getting into the eye, so that things don't seem too bright. It also cuts out badly focused light to keep vision sharp.

Its made up of a back pigment layer which absorbs the excess light, (blue eyes have less pigment). It also contains muscles to change the size of the light entry hole (pupil).

The iris muscles work against each other; one pulls the hole open (dilates the pupil), its fibers are arranged like the spokes of a wheel around a hub. The other is a circle of muscle around the pupil, when it contracts it closes the hole tight (constricted pupil).

The light reflex controls the amount of light entering the eye. If there is too much light the pupil constricts ,if there is not enough light the pupil dilates.

As well as letting light through the pupil also allows eye fluid to circulate from the back (posterior) chamber to the front (anterior) chamber. In this way nutrients in the eye fluid ( aqueous humour) can reach all the front parts of the eye.

The iris and pupil are like the diaphragm and aperture of a camera. The smaller the pupil the better the depth of field. This idea can be used to improve distance vision without glasses, by looking through a pin hole in card.

Try it yourself if distance vision is better looking through the pin hole then glasses could improve your distant vision.

Sometimes the iris can get sore, (uveitis) , if this happens then sometimes the pupil part of the iris sticks down onto the front surface of the lens, this can block the flow of the eye fluid and lead to high eye pressure and damage to the iris and optic nerve. Laser can be fired at the iris to open a hole (yag laser iridotomy) to allow the fluid to flow again.

Yag Irridotomy


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This information site has been provided by varied UK and US eye doctors for patients with eye problems.

Once your eye doctor has made a diagnosis or recommended an investigation or treatment, then you will be able to find further explanation on this site.

It is not a self diagnosis centre. It should not be relied upon without taking professional advice.