About two-thirds of the light reflected from an object in the scope of our sight is being refracted through the cornea and the other one-third is refracted through the eye lens. The cornea and the lens bent the light and focus an image of the object upon the retinal surface. The image formed on the retina is transmitted via the optical eye nerve to the brain. The travel of the refracted light in the eye is called refraction. If the light is not focused on the right spot on retina’s surface, refractive errors occur such as myopia, hypermetropia and astigmatism. People with refraction error frequently have blurry vision. Refractive errors can be ‘corrected’ with eyeglasses or contact lenses, or they can be permanently treated with LASIK and other vision correction surgery (also called refractive surgery). The eye's ability to refract or focus light sharply on the retina depends on three eye anatomy features:
1- The overall length of the eye,
2- The curvature of the cornea and
3- The curvature of the lens inside the eye.
Myopia or nearsightedness is developed when the eyeball is too long, the cornea is too steep or the lens is too steeply curved, so images are being focused in front of the retina rather than on the very retina at the back of the eye. People with myopia see near objects clearly but far away objects appear blurred. Myopia has genetic origin and also tends to be triggered by visual stress such as intensive reading or computer work. People with diabetes or cataract are also more likely to develop nearsightedness. Myopia can be treated with glasses and corrective lenses or through refractive surgery.
Hypermetropia or farsightedness, also known as hyperopia or longsightedness is developed when the eye is too short, the cornea is too flat or the lens is not round enough, so light is being focused behind the retina. As a result the eye’s lens changes shape trying to increase its refractivity and bring the image on the retina’s surface. While the ciliary muscles work too hard to ensure that the lens changes shape, a feeling of too soon eye fatigue, eye pain or headache may develop. The capability of the lens to change its shape declines as we get older. Hypermetropia has mostly genetic roots as well. People with hypermetropia can usually see distant objects quite good, while experiencing difficulties focusing on nearby objects. Corrective treatment includes prescription of glasses, contact lenses or refractive surgery.
Astigmatism develops when the cornea is not perfectly egg shaped causing the light to be refracted and focused irregularly in two axes resulting into blurred or double vision. Corrective treatment includes prescription of cylindrical glasses, contact lenses or refractive surgery. A person can be nearsighted or farsighted with or without astigmatism. The causes for stigmatism can be different such as genetic, Chalazion deformity, Keratoconus Disease, Pterygium or scars due to corneal infections.
Presbyopia is a progressing with age, reduced ability to focus on near objects caused by lens loosing its elasticity over time. Normally presbyopia develops at the age of 40-45 but it could appear earlier in people with hypermetropia. Both conditions are often mistaken due to the farsightedness due to the aging of the lens. As the focusing loss increases, prescription changes become more frequent. Presbiopia is treated by corrective Multi-Zone technique in persons with appropriate age and eye structure.