The type of lenses used in eyeglasses depends on the type of vision problem and may include:
• Concave lenses are thinnest in the center. Used to correct nearsightedness (myopia), the numerical prescription in diopters is always marked with a minus (-) symbol.
• Convex lenses are thickest in the center, like a magnifying glass. Used to correct farsightedness (hyperopia), the numerical prescription in diopters is always marked with a plus (+) symbol.
• Cylindrical lenses curve more in one direction than in the other and are often used to correct astigmatism.