Did you experience changes in your eyesight during pregnancy? I had 20/20 vision prior to my first pregnancy, and ended up needing glasses by the end. In this second pregnancy, my eyesight has gotten even worse, and I had to go for a new eye exam and purchase a new pair of glasses. My eye doctor has assured me that my vision could potentially improve after I stop nursing, but here is some information about how vision is measured. 

Learn how to interpret the ways eye doctors measure your vision. From the chart used during an eye exam to what the letters on your prescription mean, you’ll get an idea of what goes into correcting your eyesight.

Eyeglasses Prescriptions Explained: Learn how Your Vision is Measured

Ever wonder what 20/20 vision means? Confused by the numbers and letters on your eyeglasses prescription? These may seem too complicated for the average person, but you don’t have to be an eye doctor to understand them. Read on to decipher the meanings behind how your eyesight is measured.

What does 20/20 vision mean?

The 20/20 scale measures a person’s visual acuity, or sharpness of their vision. This is based on the standard eye chart, known as the Snellen eye chart, which is widely used for eye exams in the United States. Developed in the 1860s by a Dutch eye doctor, the most common variation features a large capital letter E at the top followed by ten rows of various other capital letters that get progressively smaller as they go down.

To measure eyesight, this chart is placed twenty feet away from a person’s eyes, or mirrors are used to make it appear as though it is twenty feet away. Normal vision, or 20/20 vision, means that a person can read a letter at 20 feet – the distance it’s been deemed a person should be able to read it from. If a person has 20/50 vision, this means they can see at 20 feet what a person with normal vision sees at 50 feet. Typically the fourth line of letters from the bottom denotes 20/20 vision, with the better-than-average 20/15, 20/10 and 20/5 below that.

What does my eyeglass prescription mean?

When you look at your prescription for eyeglasses, the first things you might notice are the two-letter abbreviations. These are abbreviations for Latin words.

• OS (oculus sinister): left eye
• OD (oculus dextrus): right eye
• OU (oculus uterque): both eyes

Additional abbreviations and terms included in eyeglasses prescriptions are:

• SPH = sphere: This indicates the prescription’s power and how strong the lenses need to be.
• CYL = cylinder: This denotes astigmatism and indicates how strong the lens needs to be to correct astigmatism.
• Axis: This describes the degree and direction of astigmatism.
• Add: This is an indicator of how much magnifying power is needed in a bifocal or progressive lens.
• Prism: This indicates the amount of prismatic power needed to correct eye alignment problems. Only a small percentage of eyeglasses wearers need this.

Generally, the further from zero the numbers on a prescription are, the worse the person’s eyesight is. A plus sign in front of the number indicates farsightedness and a minus sign means nearsightedness. The numbers represent diopters which is the unit used to measure the lens power necessary to correct vision.

Though a little complicated, the Latin abbreviations and numbers are used as a standardized format in an effort to make prescriptions universally readable worldwide. After your next eye exam, have your eye doctor walk you through the details of your prescription. You might surprise yourself by how much it makes sense now that you know the basics.