Eye lens. Coloured scanning electron micrograph (SEM) of the fibre-like cells that form the lens (crystalline lens) of the eye. The transparency of the lens is due to its main constituent, the protein crystallin. The cells of the lens are called fibres because of their dimensions, with lengths of 10 millimetres and diameters of 5 micrometres. The lens precisely focuses light onto the light sensitive cells at the back of the eye (retina) after it has been roughly focused by the cornea at the front of the eye. Magnification unknown.