For people who are nearsighted (negative sphere power), lens thickness means the thickest part of the lens, which is the edge, and for people who are farsighted (positive sphere power) it is the center. For nearsighted people, it will also tell you the thinnest part of the lens, which is center of the lens, and for farsighted people, it is the edge.
You can choose from the following varieties of lenses:
1. Standard plastic lenses
1.50 Standard: These are the standard plastic lenses which are currently the most commonly prescribed lens, due to their relative safety, low cost, and outstanding optical quality. Please keep in mind that these lenses are not well-suited to rimless glasses, because of their size.
2. Polycarbonate lenses
1.60 Ultra Light Polycarbonate: Polycarbonate is lighter weight than normal plastic. Hence, glasses fitted with type of lens are considerably lightweight for all day comfort and durability.
3. Aspheric lenses
1.67 extra thin: These lenses have an aspheric design that reduces chromatic aberration and edge distortion giving a better all round vision. Aspheric lenses are lighter and thus more comfortable to wear. This material is ideal for rimless glasses.
4. Ultra thin lenses
1.74 ultra thin: These have a high refractive index and hence are the thinnest and flattest plastic lens. These have a hard coating to resist scratches. This material is generally recommended for half-rim glasses.