If you wear eye glasses or contact lenses, you probably have a general understanding of your optical prescription. Perhaps your Optometrist or Optician has explained it to you, maybe you had enough interest to inquire yourself, or you simply remembered from biology class. Nonetheless, I have answered the question, "What does this mean?" numerous times from patients during the course of my work day as a certified Optometric Assistant. So, to help you sort it all out, here are some answers to help you decipher the meanings of some oft-used optical terms and symbols.
Myopia, Nearsightedness, and Minus Power are all referring to the same thing: Your eyes see well at near objects, but because myopia causes the eye to be shorter in length (the focal point is theoretically behind the retina), you need a minus power lens to focus clearly on objects in the distance. Denoted by a minus (-) sign in a written optical prescription.
Hyperopia, Farsightedness, and Plus Power are also related: Your eyes see well in the distance, but because hyperopia causes the shape of the eye to be longer (the focal point lies in front of your retina), you need a plus power lens to focus clearly on near objects. Denoted by a plus (+) sign in a written optical prescription.
Astigmatism. This condition can affect anyone: those with myopia, hyperopia, or neither. What astigmatism does is prevent the light rays from coming to a single focus on the retina. There can be many reasons for this; the most common one is that your cornea, the clear outer surface of your eyeball, is not perfectly and evenly shaped, and so it unevely changes the direction of light rays entering your eye. The result is a skewed image, regardless of what distance you are viewing it at. In an optical prescription, this is referred to as Cylinder, and is denoted by either plus or minus (+ or -) depending upon the doctor writing it, but usually is written in minus form.
Presbyopia. Literally means "old eyes" and refers to the natural aging of the eyes, which generally begins around age 40 or so. The muscles supporting the eye's lens becomes less elastic, and the lens itself is less able to change shape to focus up close (accomodation); hence the need for longer arms or, preferably, reading glasses. In a prescription, this is referred to as Add Power, and is denoted also by a plus (+) sign.
Refraction. Refraction refers to optics; your cornea and lens both provide refraction - which is the bending of light rays through a medium. When your cornea and lens fail to give you a perfect refraction - in that your vision is blurry or distorted - then extra optics (glasses, contact lenses) will be needed to give you clear vision. When the doctor arrives at your final prescription, he has performed a refraction.
D. Diopter; refers to the strength (power) of a corrective lens. Sometimes this notation is omitted on a prescription, as it is somewhat redundant (the Optician filling the prescription obviously knows that -3.50, for example, is referring to the diopter of the lens needed). However, when no correction is needed for astigmatism, the notation may read -3.50 DS, meaning diopters sphere, as clarification that there is only spherical correction needed.
OD. oculus dexter, the Latin term for Right Eye.
OS. oculus sinister, the Latin term for Left Eye.
OU. oculus uterque, the Latin term for Both Eyes.
Sph. Sphere. Shape of corrective lens surface, whether concave or convex, required to correct myopia or hyperopia.
Cyl. Cylinder. Shape of corrective lens surface required to correct astigmatism; requires a specific axis of focus (from 0 - 180 degrees). Notice that if you have no astigmatism (hence no need for a cylinder correction) there will also be nothing recorded in the axis column.
Below are examples of various written optical prescriptions:
|Rx for Myopia Rx for Hyperopia & Astigmatism Rx for Myopia/Astig./Reading|
There are other conditions that exist which require the need for prism (the symbol for which is a triangle) to help the eyes align properly. This is uncommon for most people, and so I won't go into detail about it here.
Regardless of whether you have just been given your first optical prescription, or have worn glasses most of your life, it is always a good thing to be well informed. Hopefully, this information will help you better understand your own prescription, or your child's, and will give you the added confidence to discuss your prescription, your particular condition, and your needs with your Optometrist. If nothing else, you might find yourself on Jeopardy! one day and need to know "What is presbyopia?"
Update as of March 2012: Due to the Format changes when Factoidz became Knoji, there are some formatting problems with the chart. I appreciate your patience as I try to fix the problem - thank you.
Source: My general knowledge from training and education as an Ophthalmic and Optometric Assistant
Image Source: Wikipedia