NOTE: The following material is hereby licensed for re-use under the provisions of Creative Commons CC-BY-SA 3.0.

It was originally copyright © 1998 by Daniel P. B. Smith.

On 28-Jul-2004, a modified version of it was contributed to Wikipedia under the terms of the Wikipedia License at the time, as noted here.

Draft #1, 11/14/1998. Comments to: dpbsmith_website_2006@dpbsmith.com

Disclaimer: This essay is intended to satisfy curiosity about what the notations and numbers on an eyeglass prescription mean. It is about optics, not about ophthalmology. I am not an eye doctor and this essay is not to be taken as medical advice. Questions about your own prescription should be addressed to your own eye doctor: I will not reply to mail asking such questions.

This is my eyeglass prescription.

        Rx       SPHERICAL    CYLINDRICAL    AXIS    PRISM    BASE
 ____________________________________________________________________
|       |      |            |              |       |        |       |     
|       | O.D. |   -3.25    |     -.25     |  130  |        |       |
|  D.V. |______|____________|______________|_______|________|_______|
|       |      |            |              |       |        |       |
|       | O.S. |    +.50    |    -1.00     |   80  |        |       |
|_______|______|____________|______________|_______|________|_______|
|       |      |            |              |       |        |       |
|       | O.D. |   +2.00    |              |       |        |       |
|  N.V. |______|____________|___ A D D ____|_______|________|_______|
|       |      |            |              |       |        |       |
|       | O.S. |   +2.00    |              |       |        |       |
|_______|______|____________|______________|_______|________|_______|
 
 

Rx

Abbreviation of the Latin word for "recipe."

Return to my prescription

Diopter

The strength of a lens is measured in diopters, abbreviated D.

The higher the number of diopters, the stronger the lens.

A +10 diopter lens would make a good magnifying glass.

Eyeglass lenses are usually much weaker, because eyeglasses do not work by magnifying; they work by correcting focus.

Stacking lenses combines their strength.

A +1 diopter lens combined with a +2 diopter lens forms a +3 diopter system.

Lenses come in positive (plus) and negative (minus) strengths. You can usually tell whether a lens is positive or negative by looking through it. Positive lenses tend to enlarge things when you look through them; negative lenses tend to diminish the size of things when you look through them. Because eyeglass lenses are usually weak, they don't enlarge or diminish very much.

Positive eyeglass lenses can concentrate sunlight, like a burning glass. However, they are usually much too weak to set fire to anything. This series of pictures shows the shadow cast by a pair of 1 diopter drugstore "reading glasses" outdoors in sunlight as we hold it farther and farther away from a wall. As the distance from the wall increases, the shadow of the frame seems to thicken and the bright area in the center gets smaller and brighter. It slowly changes from being "eyeglass-shaped" to circular.

Negative lenses spread sunlight instead of concentrating it.

A negative lens combined with a positive lens removes some of its strength. A -2 lens combined with a +5 lens forms a +3 diopter system.

A -3 lens stacked on top of a +3 lens looks almost like clear glass, because the combined strength is 0.

In science textbooks, positive lenses are usually diagrammed as convex on both sides; negative lenses are usually diagrammed as concave on both sizes. In a real optical system, you usually get the best optical quality when most rays of light are roughly normal to the lens surface. In the case of an eyeglass lens, this means that the lens should be roughly shaped like a cup with the hollow side toward the eye. So most eyeglass lenses are meniscus in shape.

Return to my prescription

Spherical

Usually:

• the "spherical" component is the main correction
• the "cylindrical" component is "fine tuning."

Depending on the optical setup is, lenses can act as magnifiers, lenses can introduce blur, and lenses can correct blur.

Spherical lenses act equally in all directions; they magnify, blur or correct blur the same amount in every direction.

An ordinary magnifying glass is a kind of spherical lens. When a spherical lens acts as a magnifier, it magnifies equally in all directions. In the diagram below, the magnified letters are magnified both in height and in width.

When a spherical lens puts an optical system out of focus and introduces blur, it blurs equally in all directions:

Here is how this kind of blur looks on an eye chart. This kind of blur involves no astigmatism at all; it is equally blurred in all directions.

Return to my prescription

Cylindrical

Some kinds of magnifying glasses, made specifically for reading wide columns of print, are cylindrical lenses. When a cylindrical lens acts as a magnifier, it magnifies only in one direction. In the diagram below, the magnified letters are only magnified in height, not in width.

When a cylindrical lens puts an optical system out of focus and introduces blur, it blurs only in one direction:

This is the kind of blur you get from astigmatism. The letters are smeared out directionally, as if an artist had rubbed his or her thumb across a charcoal drawing. Here is how this kind of blur might look on an eye chart:

Compare it to the kind of blur that is equally blurred in all directions. A cylindrical lens of the right strength can correct this kind of blur.

When an eye doctor refracts your eyes, usually he or she begins by finding the best spherical correction. If there is astigmatism, the next step is to remove it by adding the right amount of cylindrical correction.

Return to my prescription

Axis

Spherical lenses just have a strength; +1.0 D, -2.5D.

Here are two examples of the kind of blur you get from astigmatism. The letters are smeared out directionally, as if an artist had rubbed his or her thumb across a charcoal drawing.

A cylindrical lens of the right strength can correct this kind of blur. The second example is a little bit more blurred, and needs a stronger cylindrical lens.

But notice that in addition to being smeared more, the second example is smeared out in a different direction.

A spherical lens is the same in all directions; you can turn it around, and it doesn't change the way it magnifies, or the way it blurs:

A cylindrical lens has an axis. Turning it around so that the axis points in different directions changes the way it magnifies, and the way it blurs.

Return to my prescription

Prism

Return to my prescription

Base

Return to my prescription

D.V.

This part of the prescription describes the corrections for Distant Vision.

Most people under forty years of age do not need separate corrections for distant vision and near vision, because the lens of the eye is still flexible enough to accommodate over a wide range of distances.

With age, the lens hardens and becomes less and less able to accommodate.

This is called "presbyopia;" the "presby-" root means "old" or "elder." It is the same root as in the words "priest" and "presbyterian."

The hardening of the lens is a continuous process, not something that suddenly happens in middle age. It is occurring all along. All that happens around middle age is is that the process progresses to the point where it starts to interfere with reading.

When nursery school children want to examine something carefully, they just hold it very close to their eyes. They don't need magnifying glasses because they have such good near vision.

Return to my prescription

N.V.

"Near vision."

Return to my prescription

O.D.

(Some eyeglass prescriptions simply say "left" and "right" instead of "O.S." and "O.D.")

O.D. is an abbreviation for "oculus dexter," Latin for "right eye."

"Oculus" means eye. An eye doctor is sometimes called an "oculist" (although eye doctors themselves usually prefer to use either the term "ophthalmogist" or "optometrist").

The Latin word "dexter" means "right" (as opposed to "left"). The words "dexterity" and "dextrous" derive from this root, because the right hand is usually more skillful than the left.

Return to my prescription

O.S.

(Some eyeglass prescriptions simply say "left" and "right" instead of "O.S." and "O.D.")

O.S. is an abbreviation for "oculus sinister," Latin for "left eye."

"Oculus" means eye. An eye doctor is sometimes called an "oculist" (although eye doctors themselves usually prefer to use either the term "ophthalmologist" or "optometrist").

NOTE:

• The word "ophthalmologist" has an "h" after the "p". The first syllable is pronounced "aahf" sounds like the "oph" in "sophomore." It comes from the Greek root "ophthalmos-" meaning "eye;" ophthalmology is the science of eyes.
• The word "optometrist" comes from a different Greek root,"opto-," meaning "vision." An optometrist is one who measures vision.

The Latin word "sinister" means "left," which was also associated with the idea of evil. The "bar sinister" in a medieval heraldry meant that the bearer was of illegitimate descent.

Return to my prescription

O.D. Spherical -3.25

Return to my prescription

O.D. Cylinder -.25

Return to my prescription

O.D. Axis 130

Return to my prescription

O.S. Spherical +.50

Return to my prescription

O.S. Spherical -1.00

Return to my prescription

O.S. Axis 80

Return to my prescription

O.D. +2.00 ADD

My prescription calls for an additional 2 diopters for near vision. The lower part of my bifocals has the same optical effect as if I had bought a

Return to my prescription

Eyeglasses correct focus

As children, many of us get the idea that an eyeglass is a kind of magnifying glass. If you pick up a pair of eyeglasses and hold them at arms length, or if you look at someone else's eyeglasses as they are wearing them, what you are most likely to notice is a slight magnifying or reducing effect. But this is quite incidental. It is true that some eyeglasses may magnify a little, just as it is true that aspirin tastes sour; but we do not take aspirin for its taste, and we do not wear eyeglasses for their magnifying effect. Nothing is perfect. In addition to its intended effect, every device has other effects as well. Everything is always a compromise design. In the case of eyeglasses, the intended effect is focus correction. In addition, eyeglasses have small unwanted effects including magnification or reduction, distortion, color fringes, altered depth perception, etc.

The ideal way to correct focus would be to alter the shape of the lens of the eye itself. Next best would be to introduce a corrective lens placed as close as possible to the lens of the eye. Contact lenses, and new surgical techniques such as radial keratotomy which adjust the shape of the cornea of the eye, come close to this ideal. Return to my prescription

Not all blur is the same

Here is part of an eye chart:

Here is the way it might appear if photographed by a camera that is slightly out of focus. Notice the evenness (the letters are smeared equally in all directions) and the smoothness of the blur.

Here is the way it might appear if photographed by a camera lens that has astigmatism. The letters are smeared out directionally, as if an artist had rubbed his or her thumb across a charcoal drawing.

But here is the way it actually appears to me, when I view it through one eye, without eyeglasses:

Return to my prescription