Focus Problems
Eyeglasses work by putting a lens in front of an eye which adjusts the focal point of the eye. Most vision problems, such as Myopia (nearsightedness), and Hyperopia (farsightedness), are caused because the eye cannot properly focus an image onto the retina correctly. For example, people who are nearsighted cannot see objects clearly when they are far away, because the image focuses before it reaches the retina. Eyeglasses for people with Myopia should have a minus lens, which pushes the focus of the eye farther back.
People with Hyperopia need lenses that have a plus segment, which pushes the focal point forward. Farsightedness means that objects up close look blurred. Because Hyperopia occurs with age, many aging people buy bifocals, which have plus segments in their lenses, to help them with tasks such as reading or sewing.
Lens Shape
Lenses, like all glass, bend light. Lenses for eyeglasses are shaped in such a way that they will bend light in a certain direction. They are like two prisms smashed together. Minus lenses have thick outer edges and thin middles. The thin middles are the tips of the prisms all squashed together. This makes light spread out, moving the focal point forward.
Plus lenses are the opposite of minus lenses. They have thick middles, which is like the bases of the prisms put together, and thinner outer edges. This bends the light towards the middle, moving the focal point back. Most lenses are spherical lenses, which means the curve is smooth over the surface of the lens; the angel of the curve does not change. However, if a person has astigmatism, or a distortion in the cornea, he will need eyeglasses where the lens has been ground to have a curve that bends light along an axis. The angle of the curve depends on that person’s astigmatism, so such eyeglasses need to be customized.
Lens Power
Lens vary in strength depending on the material used to make them and the angle of the curve. The more the light is bent, the greater the strength of the eyeglasses. This bending of the light is represented by the letter D, which stands for diopters. The higher the diopter number, the stronger the lens. Lenses are also labeled by a plus (+) or minus sign (-) to signify the type of lens. Once a person has been seen by an eye doctor, he can simply look for the diopter number and sign of the type of glasses he or she needs. Sometimes, plus and minus lenses are combined to create the perfect strength of lens.