What Does My Eyeglass Prescription Mean?

One of the most common questions Optometrists receive is in regards to glasses and contact lens prescriptions and what that means for your eyes.

GlassesThe prescription that you are given at the end of your eye exam is customized for your eyes. It is determined by both the curvature of the structures in your eye and its dimensions.

Optometrists are highly trained to find a prescription that is best suited for you, and you also play a role in telling us which lenses give you the clearest vision by answering that quintessential optometry question:

“Which is clearer, 1 or 2?”

Human Eye - Cross SectionIn order to see perfectly with no glasses prescription, light rays that enter your eye need to focus exactly on your retinal tissue.

The retina is like the film in a camera, and once the light is focused correctly onto that film, the images are sent along complex pathways into your brain and you perceive the world around you.

Light has to travel through your cornea (the clear window on the front of the eye) and your lens (the clear window inside your eye), which both bend the light accordingly in order to get it focused on the retina.

The curvature of both the cornea and the lens determine your prescription, as does the length of the eye.

If the cornea and lens bend the light to perfectly land on the retina, then that person requires no eye glasses prescription and can see 20/20.

Let’s break it down into the components of the prescription:

Firstly, OD is shorthand for the Latin words Oculus Dexter and that represents your right eye. OS is shorthand for Oculus Sinister, and that represents your left eye.

The numbers themselves are split into three categories:

  • spherical
  • cylindrical (or astigmatic)
  • axis components

If you require extra magnification for reading, a fourth component, your add power, is also included.

When people say they are near-sighted, they are saying that they have reasonably good visual acuity up close, but once objects are further away, they no longer can see them with appropriate clarity. Near-sightedness is often referred to as Myopia.

Myopia - Cross Section

Myopic eyes are objectively longer than eyes that require no prescription, therefore light rays entering the eye focus too quickly, rather than directly on the retinal tissue. For myopic eyes, Optometrists prescribe minus lenses.

When people are far-sighted, or hyperopic, their eyeballs are objectively shorter than someone who requires no prescription.

Hyperopia - Cross Section

Light rays entering the eye now focus behind the retina instead of right on its surface.

These patients are able to tap into their internal focusing eye muscles to clear up their vision and move those light rays back onto the retina, allowing them to see far away without glasses.

However hyperopic patients can begin to feel eyestrain, headaches, and blur at all distances if their internal focusing system is over-worked.

Often the ability of the eye to focus through the hyperopia is limited. Therefore, Optometrists prescribe plus lenses in order to help these patients see well and comfortably.

Astigmatism is the final component of a standard prescription. Astigmatism is common – approximately 80% of people have it -and yet it is often misunderstood.

Astigmatism - Cross Section

Firstly, astigmatism is not a disease! It merely means the curvature of the structures we talked about before, the cornea and the lens, are not perfectly spherical like a basketball.

Instead, they are asymmetric, like a football.

The incoming light rays now get bent into two areas, as light hits those two planes and scatters. To correct for astigmatism, Optometrists measure a “cylindrical” component that allows the incoming light rays to focus on the retina, just like we did for hyperopia and myopia.

Astigmatism also requires an axis or degree, because the power of the cylindrical component can lie anywhere from 1 degree to 180 degrees.

If a person requires more magnification up close for reading, Optometrists include an add power.

These are often blended into the glasses with a progressive or a bifocal type lens.

This is fairly age dependent, with most people requiring some additional magnification at age 40.

Some younger people benefit from a reading prescription as well, depending on their eye position and eye teaming skills.

Our Optometrists work hard to determine the perfect prescription for your visual needs, and are available if you have any further questions about your prescription or visual health.

Book an appointment with one of our Optometrists by clicking here.

Guest post written by Dr. Siva Meiyeppen