Corrective Lens Glasses

Corrective lenses are used to correct refractive errors of the eye by modifying the effective focal length of the lens in order to alleviate the effects of conditions such as nearsightedness (myopia), farsightedness (hyperopia) or astigmatism. Another common condition in older patients is presbyopia which is caused by the eye's crystalline lens losing elasticity, progressively reducing the ability of the lens to accommodate (i.e. to focus on objects close to the eye). The power of a lens is generally measured in diopters. Glasses correcting for myopia will have negative diopter strengths, and glasses correcting for hypermetropia will have positive diopter strengths. Glasses correcting for astigmatism require two different strengths placed at right angles in the same lens. Prescription lenses, made to conform to the prescription of an ophthalmologist or optometrist, are used to make prescription glasses, which are then verified correct using a professional lensmeter.

Eyeglasses can normally correct and compensate for four types of vision deficiencies:

• Myopia is a vision disorder that causes far objects to appear blurred but near objects are seen clearly. Individuals suffering from myopia are prescribed eyeglasses with concave lenses, which compensate for the refraction error by moving the image of the distant objects that cannot be seen clearly backward onto the retina.

• Correcting hyperopia is normally done with eyeglasses with convex lenses. With this disorder, the patients can see distant objects clearly but they have trouble with seeing objects that are close to them. Eyeglasses with convex lenses compensate for the refraction errors by moving the image of a distant object forward onto the retina.

• Astigmatism is typically corrected with a cylindrical lens. This disorder is caused by a non-uniform curvature in the refractive surfaces of the eye, which leads to an abnormality in focusing the light rays on the retina. As a result, a part of the light rays are focused on the retina and the other part is focused behind it or in front of it.

• Presbyopia is more frequent in people over 40 years old and it is corrected with convex lenses. These patients need reading or bifocal eyeglasses.

Corrective eyeglasses can significantly improve the life quality of the patient as they are helpful in both correcting vision disorders and reducing problems that appear when such lenses are needed, such as headaches or squinting.