Convert Eye Prescription to 20/20 Scale

Visual Acuity and the Snellen Chart

Visual acuity (VA) is a term used to describe how sharp or clear a person’s vision is. A person’s VA reflects how well they can see fine details and differentiate objects at a certain distance. While there are different ways to measure VA, the Snellen chart is one of the most common tools eye doctors use.

The Snellen chart is an eye chart with several rows of letters. Each row represents a different level of visual acuity. Letters on the Snellen chart are arranged so the largest letters are at the top, with the topmost letter being E. As you move down each row of the chart, the letters gradually get smaller.

Visual acuity is represented as a fraction. The first number in the fraction is the distance at which the chart is viewed. Eye doctors usually hang or project the eye chart on a wall, approximately 20 feet from where the patient sits. For this reason, the first number of a visual acuity fraction in the U.S. is usually 20.

The second number of visual acuity represents the distance at which a person with normal vision should be able to read that particular line. For example, if a person has 20/20 vision, it means they can see at 20 feet what a person with normal vision should see at 20 feet. This makes a VA of 20/20 the standard for good or “normal” vision.

If you have 20/40 vision, you can see at 20 feet what a person with average vision should see at 40 feet. So, your vision is considered slightly poorer than average. But if someone has 20/15 vision, it means they can see at 20 feet what a person with normal vision would only be able to see at 15 feet. This means the person with 20/15 vision has better-than-average visual acuity.

People usually have an “uncorrected” visual acuity and a “best corrected” visual acuity. Uncorrected visual acuity is how well you see without corrective lenses. Best corrected visual acuity is how well you can see with the best corrective measures applied. In other words, it’s your visual acuity while wearing prescription glasses or contact lenses.

Keep in mind that there are other details besides VA that contribute to a person’s overall ability to see. Some examples of this are color vision, depth perception, peripheral vision, and binocular vision (how the eyes function together).

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Visual Acuity vs. Vision Prescriptions

While visual acuity indicates how sharp your vision is, a vision prescription reflects the type and level of prescription lens power needed for you to reach your best corrected VA.

Vision prescriptions are measured using diopters, which appear in quarter-increments (0.25 D, 0.50 D, 0.75 D, 1.00 D). If someone’s visual acuity is 20/20 or better, they will likely have a plano vision prescription (0.00). “Plano lenses” don’t contain any corrective power.

The farther the prescription deviates from zero — either positively or negatively — the stronger the prescription. A vision prescription with a plus sign (+) in front of it means the lenses will correct hyperopia (farsightedness). Prescriptions with a negative sign (-) in front indicate correction for myopia (nearsightedness).

A vision prescription includes all the details needed to correct your vision. The lens manufacturer uses the prescription to create the best lenses for your vision correction.

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Convert Eye Prescription to 20/20 Scale

It’s important to understand that there is no direct correspondence between a person’s visual acuity and their vision prescription. A vision prescription provides details that you don’t get from a visual acuity measurement.

To find out your visual acuity and get a valid contact lens prescription, see an eye doctor for a comprehensive eye exam and contact lens fitting. Your doctor can provide you with the right information and resources to help you get the clearest vision possible.

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