# Interpreting a prescription

Discussion in 'Optometry Archives' started by nscth, Oct 3, 2003.

1. ### nscthGuest

Can someone tell me what the "100x105" in the following prescription
for glasses means?

R-4.25-100X105
L-5.50

nscth, Oct 3, 2003

2. ### Mark AGuest

The -100 is the correction for astigmatism known as "cylinder." The -100 is
actually -1.00. The 105 is the axis of the cylinder. See below for a further
explanation.

"Sphere, Cylinder, and Axis

The three basic components of an ophthalmic prescription are sphere,
cylinder, and axis. The sphere component of the prescription is described by
lens power. Lenses with plus sphere power, correct for far-sightedness or
hyperopia, while lenses with minus sphere power correct for near-sightedness
or myopia. The spherical portion of a lens can be visualized as a slice from
the edge of a sphere; the smaller the sphere, the steeper the curve, and
thus more dramatic the power.

Often prescriptions also have a cylinder component to correct for
astigmatism. Just as the spherical component can be visualized as a slice
from the edge of a sphere, the cylinder component can be visualized as a
slice from the edge of a cylinder, parallel to the axis of the cylinder.
Light rays passing through a cylinder component will converge or diverge as
with the spherical component, however the focus will be along a line instead
of a point. This is why cylinder and axis must go hand-in-hand. As with
sphere power, cylinder power is expressed in diopters (D) and is calculated
as the inverse of the distance in meters between the focal line or meridian
and the lens. Axis is expressed in degrees between 0 and 180. Axis
measurements between 180 and 360, merely duplicate those between 0 and 180.
In other words, an axis measurement of 270 degrees is the same as a
measurement of 90 degrees, likewise a measurement of 190 degrees is the same
as a measurement of 10 degrees. So, for simplicity, axis is always expressed
between 0 and 180 degrees."
http://www.laramyk.com/learn/sphere_cylinder_and_axis.html

Mark A, Oct 3, 2003