Eyeglasses can be made out of several different types of lenses. The material used for lenses can be manufactured from glass, plastic and polycarbonate. Specific types of lenses help correct for farsightedness, nearsightedness, bifocals and other vision disorders. Understanding the differences in material, style and function can help you decide what type of lens to choose.
Material for Eyeglass Lenses
Glass was the original material used to make spectacle lenses and still is used today. The heaviest of lens material, glass can break easier but must be made to certain safety standards that will protect the eyes under normal wearing conditions.
Plastic lenses are manufactured from a material called CR-39 and are lighter and less likely to shatter, but they can scratch easier. There are coatings available that make plastic more scratch resistant.
Polycarbonate is lighter than plastic and is considered the safest lens material.
High-index lenses are made from glass or plastic and are ideal for people with strong prescriptions. These lenses are made thinner and flatter, which helps with weight and improves the optics of the high power lens.
The rule of thumb for choosing lens material is that a prescription above 4.00 diopters will get the best vision with high-index, according to eyeglasses.com.
Style and Function of Lenses for Nearsightedness
If you are nearsighted and cannot see at a distance, then you have a vision correction called myopia. These lenses are thinner in the center and thicker at the edges. If your prescription is mild, you can usually choose a lens made from any material. Stronger prescriptions and bigger frames will have heavier lenses. If you are extremely nearsighted, you are a good candidate for high-index lenses.
Style and Function of Lenses for Farsightedness
If you are farsighted and can see far away but not very close, then you have a vision correction called hyperopia. Lenses for farsightedness are thicker in the center and thinner at the edges. Heavier lenses and stronger prescriptions are usually made with plastic, polycarbonate and sometimes high-index lenses.
Lenses for Bifocals
Bifocals are used if you need help seeing in the distance and up close. These lenses come in different styles and usually are classified as line and no-line. The type without lines also is referred to as progressive or invisible. Lined bifocals have a distinct separation between your distance and near correction and when you look from far away down to read, you can tell where the bifocal begins. With no-line bifocals, the distance and near power is blended in a progression and as you look down into the bifocal, the change is more gradual. Bifocals can be made from any material.
Photochromic lenses can be made from plastic or glass and change from dark to light. They allow you to have one pair of glasses that can be used as sunglasses and for indoor use. However, these lenses do not get as dark as normal sunglasses. If you have a very strong prescription, your lenses will be thicker in the center or edges and the color variation will be very noticeable. The lenses do not get as dark in the center if you are very nearsighted. Your optician can help you determine which type of lens is best for your vision correction.