In a person with uncorrected vision, their cornea and lens refract (bend) light that enters the eye, to focus a sharp, clear image on the retina. Now if your eyes are not shaped properly (spherically) and are either too short or too long, your point of focus will not be on the correct spot within the eye, causing certain images, objects and/or distances to become blurry. These types of visual imperfections can be corrected with a custom built lens that compensates for any errors in the shape of your eyes. There are three different shapes for these lenses; cylindrical, convex and concave. Concave lenses correct nearsightedness, convex corrects farsightedness and cylindrical usually corrects for astigmatism. Glasses can also be made out of different materials. Glass lenses can be more scratch resistant; however they can sit rather heavy on your face. High-index plastics are about 20% thinner, which can make them most ideal for higher prescription. Polycarbonate plastic are the strongest available, which makes them preferred for active children.