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. The relative thickness of the lenses at the edge and at the periphery will depend on the prescription and the size of the lens (or the shape of the glasses).
You can choose from the following varieties of lenses:
1. Standard plastic lenses
1.56 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. These come with scatch resistant & Anti-reflective coating. These are more impact resistant, hence, breakage resistant.
3. Ultra thin lenses
1.67 ultra thin: These have a high refractive index and hence are the thinnest and flattest plastic lens. These have a durable hard coating to resist scratches. This material is generally recommended for half-rim glasses and if your prescripton is higher than +/-2(+/-2.25, +/-2.50..etc). These lenses shield your eyes from 100% of sun's harmful UV rays, keeping your eyes healthier for longer. These lenses reples water from lens surface, hence no smudges and easy to clean.