The Basics of Eyeglasses
The most common vision defects are myopia hyperopia astigmatism and presbyopia. Eyeglasses are a primary method of correcting refractive errors. New technologies in frames and lens materials have transformed large cumbersome eyeglasses into modern thin lightweight and fashionable accessories. Your eyecare practitioner can give you a proper lens prescription as well as help you select a pair of glasses that fit your face style and specific needs.
Lens Shapes The type of lens that you require will depend on your vision condition. The most common vision defects are myopia (nearsightedness) hyperopia (farsightedness) astigmatism and presbyopia (result of aging).
Myopia is commonly referred to as nearsightedness. Those with myopia can see up close but cannot see far away. This occurs because the eye is more elongated and the light focuses in front of and not on the retina. Myopia is corrected with a concave lens ("-" lens) that is thin in the center and thick at the edges. A concave lens allows light entering the lens to diverge so that the light will focus on the retina. Technology today has taken thick coke-bottle lenses and has made them thinner lighter safer and more cosmetically appealing than ever before.
Hyperopia is commonly referred to as farsightedness. Those with hyperopia can see far away but cannot see up close. This occurs because the eye is short and the light focuses on an imaginary point behind the retina instead of on the retina. Hyperopia is corrected with a convex lens ("+" lens) lens that is thicker in the center and thinner at the edges. A convex lens is like placing two prisms base to base. Convex lenses allow the light entering the lens to converge so that it will correctly focus on the retina. Plus lenses magnify objects. With the introduction of aspheric lens technology it is possible to minimize the bulky “bug eye” effects of plus lenses making them thinner flatter lighter and more cosmetically acceptable.
Astigmatism occurs when the cornea of the eye is not spherical causing the light to focus at various focal points and not on the retina. People with astigmatism see things longer shorter or distorted somewhat like looking through a mirror in a funhouse. Astigmatism can occur with nearsightedness or farsightedness. Cylindrical lenses (toric lenses) correct astigmatism. By being curved more in one direction than in the other both focuses are shifted to the most sensitive part of the retina to provide a sharp image.
Presbyopia occurs as we age and the crystalline lens of the eye loses its flexibility resulting in loss of close vision. Presbyopia is corrected with either a multifocal lens giving more than one usable distance or with a single plus lens worn for reading only. The single lens looks like any other plus lens. Multifocal lenses have an invisible line indicating where the distance portion ends and the near begins.