- Compound lens: a lens having both a spherical and a cylindrical component
- Cylindrical curve: a curve that radiates along a straight line, like a pipe cut lengthwise
- Diopter (D): the refractive power of a lens; the higher the number, the stronger the lens
- Refraction: the bending of light
- Spherical curve: a curve that is the same in all directions, like a basketball cut in half

## Determining Lens Strength

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:

Lens ShapesTwo 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.