OpenOptix ABO Study Guide - Laramy-K Optical

Lens Power
As light rays pass through a lens with power, the rays are bent or refracted. In a lens with
a plus power, the light rays converge or are refracted toward one another. The point at
which the light rays converge is called the focal point and in a plus lens, is behind the
lens surface. In a lens with a minus power, the light rays diverge or are refracted away
from one another. If these rays are extrapolated or traced back toward the light source,
the lines will converge and form a focal point in front of the lens surface.
The lens power is relative to the focal distance or the distance between the focal point and
the lens. More specifically, lens power is the reciprocal of the focal distance in meters.
Lens power is expressed in diopters (D).
Power = 1 / Focal Distance
As explained above, if the light rays converge, the focal distance is expressed with a
positive value resulting in a positive or plus power. If the light rays diverge, the focal
distance is expressed with a negative value resulting in a negative or minus power.
Example:
Light rays pass through a lens and converge 0.50 m from the lens. What is the power of
the lens?
Power = 1 / Focal Distance
Power = 1 / 0.50 m
Power = 2.00 D
Prism
Prism can be used to correct vision for an individual whose eyes are not perfectly aligned
as with, for example, a patient with strabismus. When the eyes are not aligned, the right
and left eye see different images resulting in blurred or double vision. Sometimes the
brain can even "shut off" one eye, in an attempt to remedy the vision, resulting in
monocular vision and loss of depth perception. Prism can sometimes be used to align the
This document is licensed under the Creative Commons Attribution 3.0 License. 7/30/2009
16