Pupil

The pupil is the round, black opening that you can see in the centre of your eye. It is situated right in the centre of the iris, the coloured part of your eye. The pupil is not actually a structure in itself, but an opening through which light passes to reach the retina at the back of your eye.

The size of the pupil is controlled by muscles in the iris. In low light, the pupil gets bigger so that more light can enter and you can see better in the dark. In bright light, however, the pupil actually gets smaller to prevent too much light from entering and causing you to be dazzled. This adjustment happens automatically and is called the pupillary reflex.

Pupil size can also change due to other factors, such as emotions, medication, tiredness or certain eye conditions. In most adults, the pupil is about 2 to 4 millimetres in size in normal light, and in the dark it can dilate to 4 to 8 millimetres.

Pupil size and contact lenses

The size of your pupil can be important when wearing contact lenses, particularly with multifocal or toric lenses. Multifocal lenses have different zones for distance and close-up vision, and a larger pupil can affect how clearly you see in different lighting conditions. With toric lenses for astigmatism, the position of the lens is important, and a very large pupil can sometimes cause halos or glare, particularly in the evening.

Pupil response and eye examinations

Your optician or ophthalmologist will often check your pupil response during an eye examination. They shine a light into your eye to see if both pupils react in the same way and if they constrict quickly enough. An abnormal pupil response can sometimes indicate an eye problem or a neurological condition, but only an ophthalmologist or doctor can assess this.

Pupil dilation during an eye examination

Sometimes, during a comprehensive eye examination, you may be given drops that temporarily dilate your pupils. This allows the ophthalmologist to get a better view of the back of your eye, for example, the retina. After such an examination, you may experience sensitivity to light and blurred near vision for a few hours. It is therefore advisable not to drive yourself and to bring a pair of sunglasses with you.

Differences in pupil size between the two eyes

In most people, both pupils are the same size and react equally to light. Sometimes there may be a slight difference; this is called anisocoria. A slight difference is often normal, but a sudden or significant difference may indicate an eye problem or other condition. If you notice anything like this, have it assessed by your optician, optometrist or ophthalmologist.