A cataract is a clouding of the lens in your eye and can develop in one or both eyes. The lens is normally clear and sits behind the iris – the coloured part of the eye. The lens helps focus light to produce a sharp image on to the retina at the back of the eye and changes shape to allow you to see close objects. A cataract acts like a frosted glass coating that scatters light, causing blurring and lack of clarity.