What are the Different Lenses for Glasses?

More than half of the people in the United States need eyeglasses or contact lenses some or all of the time, according to Prevent Blindness America. Eyeglass lenses correct vision disorders such as myopia (nearsightedness, the inability to see distant objects clearly), hyperopia (farsightedness, the inability to see near objects clearly), presbyopia (age-related loss of focusing) and astigmatism (blurred vision caused by an irregularly shaped cornea or lens).

Single-Vision Lenses

• Single-vision lenses incorporate the same prescription throughout the lens, although each separate lens in a pair of glasses may be of a different strength. When they correct nearsightedness, their shape is concave, thinnest in the center, and when they correct farsightedness their shape is convex, thickest in the center. Astigmatism correction requires a lens that curves more in one direction than the other and can be incorporated into convex or concave lenses.

Bifocal and Trifocal Lenses

• Bifocal lenses correct both distance and near vision, incorporating two lens strengths. Bifocal wearers use the upper portion of the lens for viewing distant objects and the lower portion for reading or other activities that require close-up focusing. Trifocal lenses add a third segment for viewing objects at intermediate distances. In traditional bifocals and trifocals, a distinct line separates the segments of the lens. Seamless bifocals and trifocals are polished to remove the line, which improves their appearance but causes blurred vision in the area of the polishing.

Progressive Lenses

• Progressive lenses provide vision correction at all distances by gradually changing power throughout the lens. Some people experience difficulty in adjusting to progressive lenses and find their vision is blurred when not looking directly through the center of the lens. Progressive lenses are more expensive than lined bifocals or trifocals.

Non-Prescription Reading Glasses

• Most drugstores sell non-prescription reading glasses in a variety of magnification strengths. People with presbyopia wear these glasses only for reading or other close-up viewing, often alternating them with prescription eyeglasses for nearsightedness.

Optional Features

• Glass lenses scratch less easily but are heavier. Most modern eyeglass lenses are impact-resistant plastic, with special coatings added to help prevent scratching. Optional anti-reflective coatings reduce glare when using a computer or engaging in outdoor sports. Photochromic lenses remain clear indoors but darken in sunlight to protect the eyes from ultraviolet radiation, making prescription sunglasses unnecessary.