Did you know that when a person has 20/20 vision, that means they have normal visual acuity measured at a distance of 20 feet? They can see clearly at 20 feet what should normally be seen at a distance of 20 feet. Most people are relatively familiar with this method of representing visual clarity or sharpness but don’t fully understand how to read a glasses prescription. If you’ve always wondered what those numbers and abbreviations on your prescription represent, scroll down for a quick guide.
How to Read a Glasses Prescription
OS and OD
Some of the first things you may notice when you view your eyeglasses prescription are the headings OS and OD. Sometimes these will have a helpful label to convey their meaning, but if not, here’s what you need to know:
- OS is an abbreviation of oculus sinister, which is Latin for “left eye.”
- OD is an abbreviation of oculus dexter, which is Latin for “right eye.”
- OU, an abbreviation of the Latin oculus uterque, which stands for “both eyes,” is occasionally used as well.
Since most people don’t understand Latin or know the abbreviations OS and OD, some optometrists will write “Right” or “RE” (right eye) and “Left” or “LE” (left eye) instead.
The information for the right eye is listed before the information for the left eye because when the optometrist is facing you, they read your eyes from left to right (which means your right eye comes first).
The numbers you see represent diopters, the units used to measure the correction of the lens (the focusing power) required by each of your eyes. Sometimes diopter is abbreviated to “D.”
On a glasses prescription, 0.00 (zero focusing power needed) represents good vision. Generally, the further away from zero you go (whether the number is positive or negative), the worse your eyesight and the greater the need for vision correction. So +1.00 and -1.00 are quite modest; your eyesight isn’t too bad, as you only need 1 diopter of correction. On the other hand, +4.50 and -4.50 represent a greater lack of clarity; you’ll need a stronger prescription, at 4.5 diopters of correction.
Plus (+) and Minus (-)
So what’s up with the positive and negative numbers?
- A plus sign (+) in front of a number means you are farsighted (i.e., things up close are blurry).
- A negative sign (-) in front of a number means you are nearsighted (i.e., things far away are blurry).
If you suffer from astigmatism, your cornea or lens is irregularly shaped, which causes blurred vision. Your optometrist will measure your astigmatism to ensure that your prescription accounts for it. Typically, this is detailed with three numbers:
- Spherical (S): This is the degree of nearsightedness or farsightedness measured in diopters, as described above. All prescriptions will include spherical information.
- Cylinder (C): Like visual acuity, this is measured in diopters and can be positive or negative. It relays the degree of astigmatism. The bigger the number, the more significant the astigmatism. If you don’t have astigmatism, this section will be left blank or won’t be included.
- Axis: This is the orientation of the astigmatism, and it can be anywhere from 0 to 180 degrees (90 corresponds to the eye’s vertical meridian, and 180 corresponds to the eye’s horizontal meridian). It shows where the curvature in the cornea or lens is taking place. If your prescription includes cylinder data, it will also include axis data, as both are needed for a prescription for someone with astigmatism.
Most people begin to suffer from presbyopia (a loss of focusing ability) as they age. For example, they may have trouble reading the menu at a restaurant or text messages on a phone. If you suffer from presbyopia, your optometrist may also include an “add,” which is the added magnifying power applied to the bottom of multifocal lenses to assist with presbyopia.
If your eyes are not aligned, your optometrist will also include a “prismatic power,” measured in prism diopters (p.d.), to compensate for the lack of alignment. Only a small percentage of glasses prescriptions will include prism. It’s measured in metric or fractional English units, and the direction of the prism is indicated in one of four ways relating to the position of the base: base up (BU), base down (BD), base in (BI), base out (BO). Base in means toward the nose, while base out means toward the ear.
Finally, your optometrist may include a specific lens recommendation on your prescription: progressive lenses, photochromic lenses, lens with anti-reflective coating, etc.
If you have any questions about how to read a glasses prescription, don’t hesitate to ask your optometrist.
Looking for a new optometrist? If you live near southwest Missouri, stop by Heffington’s. Since 1975, the Heffington family has been assisting the Springfield community with top-quality eye care and affordable eyeglasses and contacts. To learn more about our products and services, please get in touch with us online, send an e-mail to [email protected], or give us a call at 417-869-3937 (Optiland location) or 417-882-3937 (House of Vision location).