OpenOptix ABO Study Guide - Laramy-K Optical

Of course, most lenses are not comprised of angular prismatic surfaces but consist of
curved surfaces. The most basic of these curves is a sphere. The curve on the surface of a
spherical lens, if extrapolated in all directions, would form a ball or perfect sphere. The
sphere would vary in size based on the steepness of the curve. A steeper, higher power
curve would form a smaller sphere with a smaller radius, while a flatter, lower power
curve would form a larger sphere with a larger radius.
In addition to being described by their power or radius, spherical curves also have a
direction. An inward curve is called concave, while an outward curve is called convex.
Thinking back to the prism example, a minus lens that diverges light would require a
concave spherical surface, while a plus lens that converges light would require a convex
surface. Therefore, we use the minus (-) sign to denote concave curves, the plus (+) sign
to denote convex curves, and the term "plano" to describe a flat or zero curve.
A lens has two curved surfaces of consequence to the vision of the wearer: the front
surface and the back surface. Common lens shapes based on front and back curves are
described in the figure below.
This document is licensed under the Creative Commons Attribution 3.0 License. 7/30/2009
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