What is a cataract?

Inside the eye is a small lens that focuses light so that we can see clearly. In young people it also allows the eye to change focus so that we can see near and far. Cataract is when the lens loses clarity and the vision is affected. A lens that has lost clarity, or become cloudy, can have a variety of effects on vision.

Cataracts: introduction


Often the vision will become blurry for distance vision, or for reading.

Dim vision with loss of colour brightness

This particularly affects blue colours.


Sometimes the vision can be quite good in good light, but when light shines towards the eye (a low sun or oncoming car headlamps) vision becomes grey and the contrast becomes poor; this is called glare.

Changing focus

Some cataracts change the strength of the human lens and so cause a change in the focus of the eye. Early focus change can often be dealt with simply by changing your glasses. As the cataract progresses the speed of change of focus may become so rapid that simply updating your glasses is no longer an effective way of dealing with the problem.

Multiple images or ghosting of images

These can be caused by light scatter within the cloudy lens.

Cataracts develop as part of normal ageing: at least a third of people over 65 will have enough cataract in at least one eye to reduce the vision below the driving standard. Some diseases, drugs and injuries can also cause cataract.

The word cataract comes from the Greek "kataractes" meaning something rushing down. The other meaning is a large waterfall. Advanced cataract can make the lens white and give it an appearance like water crashing down in a forceful waterfall.

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