Criteria for LASIK
- Have a stable prescription for glasses for at least 1 year.
- Have no progressive retinal disease.
- Glaucoma must have been excluded.
- Be at least 18 years of age
What is defective vision?
In defective vision, incoming light rays and/or pixels are not precisely focussed onto the retina at the focal point like in eyes with “normal” vision. The result is a blurred or distorted picture, as the ratio between axial eye length and corneal curvature is frequently incorrect. In principle, you can compare the eye to a (photo) camera: Cornea and lens are the equivalent of the lens, and the retina is equivalent to the film.
In short-sightedness (myopia), close objects can be seen clearly, while distant objects appear blurred: The eye is too long; the light rays are already focused in front of the retina. In laser eye correction, the refractive power of the cornea is reduced by a certain amount, for example, -2.00 Dioptres, and the focal point is shifted to the retina.
In long-sightedness (hyperopia), close objects appear blurred, while distant objects can be seen clearly. The light rays only meet once they hit the retina. Long-sightedness can be compensated for a long time, as the lens will adjust. It increases the refractive power and focuses the light rays on the retina. Long-sighted people are usually able to retain good long distance vision into old age. However the ability of the lens to adjust, diminishes with age, close objects start getting blurry, which means long-sighted people will need reading glasses earlier.
In astigmatism, objects at all distances appear distorted, because the cornea is curved like an eggshell and not like a ball, and the lens therefore does not have enough refractive power.
Most people with normal sight need reading glasses by the age of 45, for the simple reason that the lens loses its elasticity and it becomes less capable of “bending”. Once eyes gradually lose the ability to adjust, objects at reading distance appear blurred. One starts holding reading text further away or resorts to reading glasses. However, this natural ageing process does not compensate for the short-sightedness of a young adult. Someone who is short-sighted may be able to read without glasses even in old age, but will always need distance glasses, e.g. for driving.