If your eye doctor determines you need corrective lenses, he or she will write an eyeglasses prescription for you at the end of your exam.  This prescription specifies the lens powers required to correct your vision.

A prescription for eyeglasses cannot be used to purchase contact lenses.  Contact lens prescriptions contain additional information that can only be determined during a contact lens fitting. 

At a minimum, your eyeglasses prescription will contain the following information:  your name, the prescribing doctor's name, the date of the exam, and the sphere power, cylinder power, cylinder axis, prism power and orientation, and add power of the lenses.

Your prescription may contain the Latin abbreviations OD (oculus dextrus) and OS (oculus sinister) to specify lens values for your right eye (OD) and left eye (OS), respectively.  In some cases, the English abbreviations RE (right eye) and LE (left eye) are used instead.

Figure 1.  Sample Eyeglasses Prescription Form

Eyeglasses Rx for: __________________________________________
	Sphere	Cylinder	Axis	Prism	Add
OD
OS
Dr. ________________________________   Exam Date ____________

Sphere Power

The sphere power indicates the lens power prescribed to correct nearsightedness or farsightedness.  Sphere powers are measured in diopters (D).  Eyeglass lenses are normally prescribed and manufactured in one-quarter diopter (0.25 D) increments. 

If a sphere power has a minus sign (-) preceding the diopter value, the lens is a concave lens for the correction of nearsightedness.  Concave lenses (also called minus-power lenses) are thicker at the edge of the lens and thinner at the center.

If a sphere power has a plus sign (+) preceding the diopter value, the lens is a convex lens for the correction of farsightedness.  Convex lenses (also called plus-power lenses) are thicker at the center and thinner at the edge.  Convex lenses are also used in reading glasses and are added to the bottom portion of bifocal, trifocal, and Progressive lenses to correct Presbyopia.

Some eye doctors will write the abbreviation "sph" after the sphere power when an eyeglasses prescription contains sphere power only.

Cylinder Power

The cylinder power indicates the lens power prescribed to correct astigmatism.  If you have astigmatism, the cornea of your eye is shaped like the back of a spoon (more curved in one meridian than in others).  Cylinder power is the difference in sphere power between the steepest and flattest meridians of an eye with astigmatism.  It is called cylinder power because it has no curve (or power) in one meridian, and its maximum curve (and power) is located at a 90-degree angle to the meridian with no power.  Hence, it is shaped like a cylinder.  The meridian of the cylinder power that has no power is called the cylinder axis.

Like sphere power, cylinder power is measured in diopters (D), in 0.25 D steps.  Cylinder power is usually specified in minus-power format, but may be written in plus-power format as well. 

If you have no astigmatism, the cylinder power and cylinder axis columns of your prescription will be left blank.

Cylinder Axis

The convention used for describing the location of cylinder power in lenses is a scale of 180 radially-arranged meridians on the surface of the cornea. (Imagine a protractor scale superimposed on your cornea.  Each meridian represents a 1-degree angular increment on the scale.)

The first meridian (001) is a nearly-horizontal line tilted slightly above horizontal on the right side of the eye.  The next meridian (002) is a line rotated slightly counter-clockwise from the 001 meridian.  As you continue to move in a counter-clockwise direction, meridian values increase until you reach the 180 meridian.  The 90-degree meridian is vertical; the 180-degree meridian is horizontal.  This meridian convention is oriented the same way for both eyes.     

 Most eye doctors use three digits to describe all meridian locations (e.g. "001" instead of "1"; "090" instead of "90"). 

The cylinder axis value on your eyeglasses prescription is the lens meridian that contains NO cylinder power.  The cylinder power on your prescription is located at the lens meridian that is 90 degrees away from the cylinder axis.

In an eyeglass lens prescription that has both a sphere power and cylinder power, the two powers are added together to create what is called a spherocylindrical lens prescription.

Example:  Your eyeglasses prescription is:  -2.00 -0.50 x 090

This prescription has a sphere power (-2.00 D) and a cylinder power (-0.50 D).  The cylinder axis is located at 90 degrees (the vertical meridian).

Since the axis value is the lens meridian where there is NO cylinder power, The total power in the vertical (90 deg.) meridian of the lens is -2.00 D (Sphere power + cylinder power = -2.00 D + 0.00 D)

Since the cylinder power is located 90 degrees away from the cylinder axis, The total power in the horizontal (180 deg.) meridian of the lens is -2.50 D (Sphere power + cylinder power = -2.00 D + -0.50 D)

Prism

In some cases, your doctor may prescribe prism power to help your eyes work more efficiently as a team an/or stay in proper alignment. 

Prism is added to an eyeglass lens prescription only if a problem with eye teaming or eye alignment is detected.

Prism is measured in special power units called prism diopters (p.d.).  Unlike the diopter units used for sphere power and cylinder power, prism diopters are usually expressed in fractions (e.g. 1 1/2) rather than in decimal units.

If prism power is prescribed, a second value called the prism base (or simply, base) is included to indicate the direction (in, out, up or down) of the prism in the lens. 

Your eye doctor may prescribe base-in prism if your eyes have a tendency to become misaligned in an outward direction.  If your eyes tend to cross inward, base-out prism may be prescribed.  In some cases, lateral prism (base-out or base-in prism) is prescribed for eyestrain.

Lateral prism is always prescribed in the same direction for both eyes (i.e. base-out prism for both eyes or base-in prism for both eyes).  Sometimes lateral prism is prescribed for one eye only.

Vertical prism (base-up and base-down prism) is used for vertical eye misalignments or difficulties in maintaining the eyes in comfortable vertical alignment. Vertical prism is prescribed in opposite directions for the two eyes (base-up prism for one eye and base-down prism for the other).  Sometimes vertical prism is prescribed for one eye only.  

Add Power

If you have presbyopia or other near focusing problems, your doctor may write an eyeglasses prescription that includes an add power.  The add power is extra magnifying power added to the lower part of your lenses so you can see near objects more clearly.

Lenses used for add powers are always spherical plus-power lenses, typically within the range of +1.00 to +3.00 D.  Add powers are nearly always equal for both eyes.

Lens prescriptions that include an add power can be used for bifocals, trifocals, and progressive ("no-line bifocal") lenses.

Expiration Date

In addition to noting the date of your exam, your eye doctor may include an expiration date on your eyeglass lens prescription. 

Over time, changes can take place in your eyes that affect your vision very gradually.  You may not be aware that your eyes have changed and your eyeglasses are no longer giving you your best vision. 

For your own safety, see your eye doctor on a regular basis for routine exams.  Do not attempt to use an expired eyeglasses prescription to purchase eyewear.