Many people have 20:20 vision but this depends on three things; the curvature of your cornea, the focusing power of your lens, and the length of your eye. Any imbalance between these three will result in a focusing / refractive error. Refractive errors are measured in Dioptres (D).

There are three main types of refractive errors - short-sightedness (myopia), long-sightedness (hyperopia) or astigmatism. In these eyes the focusing power does not form a clear image on the retina (photographic layer) of the eye. The focusing / refractive error can be corrected by glasses or contact lenses to give clear vision. The optician's prescription is written as below:

[+ / - Sphere] [+ / - cylinder] [axis]

Normal Vision

Short-sightedness (Myopia)

Myopia is when near objects are seen clearly but those in the distance are blurred. It occurs if the cornea is too steeply curved or if the eye is slightly longer than normal so that light is focused in front of the retina. Without glasses or contact lenses, short-sighted people are able to see objects clearly if held close to their eyes, but usually those objects beyond arms length are blurred or out of focus. This is a common eye problem affecting approximately one in five adults. Myopia is indicated with a minus sign (-) on prescriptions. This refractive error can be corrected by laser and implants.

Long-sightedness (Hyperopia)

Hyperopia occurs when the cornea is too flat for the size of the eye. This time light is focused beyond the retina so that close objects are blurred. Depending on their age, long sighted people may also have difficulty focusing on distant objects as well. Hyperopia is indicated with a plus sign (+) on prescriptions. This refractive error can be corrected by laser and implants.

Astigmatism

Astigmatism is when the eye has two different degrees of long (or short) sightedness combined. For example, vertical objects may be clear but horizontal ones blurred. No amount of focusing can give a completely clear image. Most people have a degree of astigmatism, which may co-exist with myopia or hyperopia. This is indicated as the cylinder (cyl) and will either have a + or a - sign in front of a number. This refractive error can be corrected by laser, implants and corneal microsurgery.

Presbyopia

This is the need for reading glasses that occurs when the eye becomes unable to focus on close objects. It is a natural ageing process that affects everyone between the ages of 40 and 50 years. This often co-exists with other refractive errors and can be corrected by implants such as accommodating implants (see implants) and monovision laser treatments.

Keratoconus

In this condition, the cornea is not a spherical surface / round but is distorted due to thinning of the cornea. This leads to unusual refractive errors with high amounts of astigmatism and may need complex glasses or hard contact lenses to improve vision. In its extreme form, the cornea becomes conical in shape. Certain types of keratoconus can be treated with specialised implants (Intacs/Ferrara) within the cornea to reduce the distortion allowing better vision with glasses or contact lenses. Some keratoconic eyes develop superficial scarring which prevents good vision and this scarring can be removed with the Excimer Laser.

For more information about Intacs click on image below