Contact lenses, like glasses, are designed to correct your vision. They are generally no bigger than a 5 pence piece, and are typically made from a soft and flexible material. Contact lenses float on the layer of natural tears which covers the surface of the eye. This is why you hardly notice you’re wearing them.
Contact lenses correct vision by compensating for refractive errors and focusing the image properly, so your sight is clear and sharp. Vision is blurred when light is not focused properly on the retina (the part of your eye that reads the image, like the film in a camera).
- Astigmatism (rugby ball-shaped eyes)
- Presbyopia (ageing eyes)
A shortsighted (myopic) eye focuses the light in front of the retina, and needs a lens with a minus (‘-‘) prescription. To correct myopia, the contact lens is thinnest at the centre. This moves the focal point of the light backwards, so that it reaches the retina.
A longsighted (hyperopic) eye focuses the light behind the retina, and needs a lens with a plus (‘+’) prescription. To correct hyperopia, the lens is thinnest at the edges. This brings the focal point forwards so that light is focused on, rather than behind, the retina.
With astigmatism, the cornea is shaped more like a rugby ball, instead of a spherical football shape. This causes the light to bend unequally, producing two focal points in front of or behind the retina. Toric contact lenses have different curvatures which correct vision by creating a single focal point on the retina.
Presbyopia is a natural process that occurs with age. Over time there is a gradual thickening and hardening of the lens, so its ability to focus, particularly close up, steadily decreases. Where presbyopia requires both near and far correction, you need bifocal, or multifocal contact lenses. There are several types of multifocal lens available. Some have near vision correction in a small circle at the centre, and distance correction surrounding it. Some mimic bifocal glasses, with distance correction in the top half, and near in the bottom. Some place both near and far correction near the centre of the lens.