Reading an Eyeglass Prescription

If you wear prescription glasses, chances are you’re used to having eye exams, but did you know your doctor is required by law to give you a copy of your eye prescription? If you take the time to understand how to read it, you can learn a lot about your eyes: whether you’re near or far sighted, if you have an astigmatism, how much your vision needs correcting, and even if you need bifocals. That’s a lot of information from a few abbreviations and some numbers.

Deciphering the Letters

One of the first things you’ll notice when you look at your prescription is the letters “OD” and “OS.” These are abbreviations that represent the Latin terms for your left and right eye. The right eye is known as “oculus dexter” and the left is known as “oculus sinister.”  You may also see “OU,” or “oculus utereque,” which refers to both eyes.

“Oculus” is the Latin word for eye. For OD and OU, the explanations for the names are pretty simple and straightforward. “Dexter” is the Latin word for “right” and “utereque” for “both.”  It might seem curious, though, that the word “sinister” is used to describe the left eye. This is because the left side of anything was considered to be unlucky or unfavorable.

There are also a few other abbreviations that show up on your eye prescription. “SPH” stands for sphere, which is the amount of power needed to correct the eye’s vision. To figure out what this value should be, your eye doctor will determine whether you are nearsighted or farsighted.

If an eye has normal vision, light comes through the lens and is focused directly onto the retina. Nearsightedness and farsightedness occur when the light isn’t focused correctly. In nearsightedness, it’s focused in front of the retina. In farsightedness, it’s focused behind it.

If you’re nearsighted, you can see things that are close to you better than you can see things far away. Nearsighted people will have a negative sphere value. Farsighted is the opposite. Objects in the distance are clear while those close by are blurry. To correct this, you’ll need a positive sphere. The lenses change the way the light is focused and move it back to the retina.

The letters “CYL” may appear, too. This is the abbreviation for “cylinder” which is the measure of what is needed to correct and astigmatism. An astigmatism is a flaw in the way your cornea curves. Your cornea covers the colored part of your eye and your pupil. Normally, it is uniform in all directions. If it isn’t, your eye can’t refract light as precisely which will affect you vision.

Lens power is added to correct an astigmatism. This measurement is also positive to correct farsightedness and negative to correct nearsightedness. Your CYL measurement will always be the second one listed on your prescription as SPH always comes first.

Understanding the Numbers

Of course, your eyeglass prescription includes numbers, too.

Sphere values indicate how much correction each eye needs to get your vision to 20/20. The unit of measurement is a diopter. The further away from zero this number is, the more correction that eye needs from your prescription glasses. For example, if your measurement is -2, you have 2 diopters of nearsightedness. If it’s +3, you have 3 diopters of farsightedness.

If you have an astigmatism, you’ll see an axis value on your prescription. This number indicates where on the eye the astigmatism is so that is can be targeted and corrected. It’s quite interesting how this number is obtained because you might assume it would be a more complicate method than what it really is.

Imagine a protractor placed over your eye with 0 degrees on the outer corner and 180 degrees on the inner one. This is where the axis measurement comes from. The value on your prescription will be a number from 0 – 180 to communicate where on your eye the astigmatism is located. It’s not enough for the prescription to identify and measure the astigmatism. In order to correct it, the person making your lenses has to be able to identify exactly where on the eye it’s located. This “protractor system” is a simple but effective way to do just that.

If you need multifocal lenses, you’ll also see a number marked “add.” This is always a positive value because it’s adding more power to the bottom of the lens of both eyes. These types of lenses are used to treat presbyopia and are commonly otherwise known as bifocals. As people age, they often have a hard time seeing small print, reading, or seeing computer screens clearly. Multifocal lenses correct this.

Using Your Prescription

When you use your eyeglasses prescription to get glasses, there’s one more measurement they’ll need: pupillary distance. This is the distance between the center of each pupil and is measured in millimeters. This isn’t a measurement or assessment of you vision. Pupillary distance, or PD, just tells the manufacturer where to put the center of the lenses in your new glasses so you will be able to see properly.

In addition to getting your standard glasses, an eyeglass prescription can also be used to get prescription sunglasses. It’s important to note that it cannot be used for contact lenses. There are a lot of reasons for this, but the easiest way to think about it is that the lenses of your glasses sit right in front of your eyes while contact lenses actually sit on them. There are different measurements that need to be taken. You won’t need to schedule an additional exam, though, they can usually be done at the same time.

Knowing Your Eyes

Whether you’re in the market for a new pair of your everyday glasses or something more specialized like prescription safety glasses, your doctor is required by law to provide you with your prescription after the exam. By taking the time to understand how to read it, you’ll learn a lot about your eyes and get a better understanding of how your glasses work.  


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