The strength of a lens is determined by the lens material and the angle of the curve that is ground into the lens. Lens strength is expressed as diopters (D), which indicates how much the light is bent. The higher the diopter, the stronger the lens. Also, a plus (+) or minus (-) sign before the diopter strength indicates the type of lens.
Plus and minus lenses can be combined, with the total lens type being the algebraic sum of the two. For example, a +2.00D lens added to a -5.00D lens yields:
Two basic lens shapes are commonly used in optometry: spherical and cylindrical.
- A spherical lens looks like a basketball cut in half. The curve is the same all over the surface of the lens.
- A cylindrical lens looks like a pipe cut lengthwise. The direction of a cylinder curve's spine (axis) defines its orientation. It will only bend light along that axis. Cylinder curves are commonly used to correct astigmatism, as the axis can be made to match the axis of the aberration on the cornea.