Vision Problems and Prescriptions
What is what?
Is presbyopia when you cannot see near or far? Is strabismus contagious? I hear about progressive lenses, but what are they?
Here are some definitions explaining what is what in the optical field.
Myopia is a decrease of visual acuity in distance vision. The near vision however is not affected. Myopia causes the image focus to be in front of the retina thus creating a blurry image. A concave lens, thicker on the sides than in the middle, will bring the focus back on the retina thus creating a clear image.
Hyperopia is a decrease in visual acuity at all levels but particularly in near vision. Hyperopia causes the image focus to be behind the retina thus creating a blurry image. A convex lens, thicker in the middle than on the sides, will bring the focus back on the retina thus creating a clear image.
Hyperopia is the most frequent vision problem in children and must be corrected as soon as possible.
Astigmatism can appear with myopia, hyperopia, and presbyopia. It is caused by a deformation of the cornea which causes the focus to be made in two different points. Both near vision and distance vision are thus dramatically affected. It is difficult to trace parallel lines or to tell if a frame is crooked. A toric lens, which side thickness differs from one axis to the other, will bring the focus back on the retina thus creating a clear image.
Presbyopia is caused by the natural aging of the crystalline lens and ends up affecting most of us. With age, the lens becomes less flexible and cannot make the necessary compensations for near vision. The image focus is behind the retina thus creating a blurry image. A convex lens, thicker in the middle than on the sides, will bring the focus back on the retina thus creating a clear image.
Strabismus, better known as cross-eyed, is a muscle defect causing an imbalance in muscular strengths. Strabismus is convergent when the eye looks inward and divergent when the eye looks outward. Since the brain discards the image sent by the affected eye, there is some decrease of vision zone.
Strabismus usually appears around the age of four. It may be treated by making the affected eye “work” by blocking the healthy eye with a patch. Special eyeglasses or surgery may also be used as treatment.