For people who feel they have poor or even “low vision,” a pair of new glasses can be life-changing. The difference between someone’s vision, when they aren’t wearing their glasses versus when they are, is pretty magical. But how can a little piece of plastic make such a difference in a person’s ability to see?
Every corrective lens is made specifically to match the wearer’s vision prescription so they get the best correction possible. To understand how lenses correct vision, you must first understand what causes blurry vision.
Below, we cover the basics of what needs to “go right” within the eyes to achieve clear vision. We also explain the most common causes of blurry vision and how corrective lenses in glasses work to help make vision more clear.
How the Eyes Work to Achieve Clear Vision
There are many more parts to the eye than what you can see when you look in the mirror. Different chambers, structures, muscles, and tissues are constantly working together to keep your eyes healthy and your vision clear.
Clear vision begins with the cornea — the clear, outer layer of the eye — and the tear film — the smooth layer of fluid on top. Light rays pass through the tear film and cornea as they enter the eye. For the light rays to focus correctly within the eye, the cornea must be perfectly smooth and the tears should be distributed equally across the eye’s surface without dry patches.
After light passes through the cornea, it’s filtered through the pupil. The iris (the colored part of the eye) is a tissue that surrounds the pupil and controls how much light enters the eye. Too much or too little light can impact how well you see, so muscles within the iris adjust the pupil’s size to make sure the right amount of light gets through.
Once funneled by the pupils, light passes through the crystalline lens of the eye. The lens and the cornea are a tag-team that makes sure light is focused properly. If there’s a problem with one or both of these structures, blurry vision can result.
When everything goes right, light rays should focus perfectly on the retina like a spotlight on a stage. The retina is a tissue at the back of the eye that’s full of light-sensitive cells called photoreceptors. Photoreceptors give us color vision, night vision, and other critical parts of eyesight by turning light rays into electrical signals that are sent to the brain.
The optic nerve is attached to the retina and works as the messenger between the eye and the brain. It takes the electrical signals created by the photoreceptors and sends them to the brain. Once in the brain’s occipital lobe, these electrical impulses are processed to form an image.
Understanding Refractive Errors
When you consider all the things that have to line up for your eyesight to be sharp, it’s easy to understand why blurred vision is so common. The most common eye issues that cause blurred vision are called refractive errors.
Refractive errors are problems within the eye that affect its ability to bend light. Remember the comparison of the spotlight on a stage? With refractive errors, rather than the spotlight focusing on the middle of the stage, it’s focused in front of it, behind it, or scattered around it.
Types of Refractive Errors
There are four kinds of refractive errors. While they affect the eye differently, they all cause blurry vision at varying distances. Refractive errors include myopia, hyperopia, presbyopia, and astigmatism:
Myopia
Also called nearsightedness, myopia is a refractive error that affects your ability to see objects at a distance. It occurs when the shape of your eyeball is longer than average, making it more oblong than spherical.
Because of the eyeball’s shape and length, incoming light rays fall in front of the retina. In other words, the spotlight hits in front of the stage rather than in the middle of it. A person with myopia can see things clearly up close, but their distance vision is blurry.
Hyperopia
Farsightedness is a vision problem that affects close-up vision. It happens when the shape of the eyeball is shorter than average. The shortened eyeball length causes light to focus past the retina (or behind the stage in the spotlight analogy).
Farsightedness requires the crystalline lens of the eye to work extra hard to help focus. When the crystalline lens does extra focusing of light, it’s called accommodation.
Someone who has hyperopia can see distant objects clearly, but when the crystalline lens is unable to accommodate adequately, close-up objects appear blurry. This can make it difficult to perform tasks like reading a book, a menu, or an app on your phone.
Presbyopia
Like hyperopia, presbyopia affects near vision. However, the two issues are caused by different things. Presbyopia has nothing to do with the shape of the eyeball. Rather, it’s caused by age-related changes in the eye’s crystalline lens.
Similar to the lens of a camera, the lens of the eye adjusts to help focus light in the right direction. Once you reach age 40, the eye’s natural lens stiffens and becomes less flexible, affecting its ability to focus. This loss of focusing ability impacts a person’s near vision. It’s also why it’s common to see older adults using reading glasses.
Astigmatism
While astigmatism can be caused by abnormalities with the lens, it’s usually caused by problems in the cornea. As mentioned earlier, the front surface of the eye must be perfectly smooth for light to focus correctly. With astigmatism, there’s an oval shape in the curvature of the cornea.
Since the cornea is the first checkpoint for incoming light, an oval shape in the cornea can affect the light’s point of focus from the very beginning. With astigmatism, a corneal abnormality causes incoming light to focus in two spots instead of one. It’s as if you’re pointing two spotlights on the stage with neither pointed directly at the performer.
This can make your vision blurry at all distances (near, far, and intermediate), and it can make night driving particularly difficult. Astigmatism can occur on its own, or in addition to one or more of the other refractive errors described above.
How Do Glasses Improve Vision?
To put it simply, prescription glasses work by redirecting light so that it hits the retina in the correct place. The type of refractive error(s) you have determines the type of corrective lens you’ll need.
Types of Corrective Lenses
What makes lens types different — besides the refractive error they correct — is where the thickest and thinnest parts of the lens are.
It’s possible for each of your eyes to have different corrective needs, which can mean a different lens type for each eye.
Lenses to Correct Myopia
People with myopia need concave lenses to correct their nearsightedness. A concave lens is thinner in the center and gets thicker toward the edges.
Glasses with concave lenses work by manipulating light rays entering the eye so they come to focus farther back than normal. Doing so makes the light focus perfectly on the retina since a myopic eye is longer than average.
Lenses to Correct Hyperopia
Convex lenses are prescribed to people who have hyperopia. In a convex lens, the thickest area is in the middle of the lens and the edges are thin.
Glasses with convex lenses work by redirecting light so it focuses closer up in the eye. Because a hyperopic eye is shorter than average, this redirection of light allows it to focus directly on the retina.
Lenses to Correct Presbyopia
Convex lenses are also used to treat presbyopia, since hyperopia and presbyopia both affect near vision. However, convex lenses take a different approach to correcting presbyopia.
As mentioned earlier, presbyopia is caused by age-related inflexibility of the eye’s natural lens. To help correct blurry near vision in people with presbyopia, convex lenses focus light before it reaches the natural lens.
The convex lenses compensate for the poor focusing ability of the natural lens and restore sharp close-up vision. Essentially, the corrective lens does the accommodating (extra focusing of light) for you.
Lenses to Correct Astigmatism
Astigmatism is corrected with a cylindrical lens. (The convex and concave lenses we just discussed are considered spherical lenses.)
There are a few key differences between cylindrical lenses and spherical lenses:
- Spherical lenses focus light into a single point.
- Cylindrical lenses focus light into a specific line located between two points of focus.
- Spherical lenses have the same vision prescription throughout the entire lens.
- Cylindrical lenses have a corrective power that varies across the lens because one meridian (axis) of the lens has a different curvature than the other.
In most people, astigmatism is caused by the oval shape in the curvature of their cornea. The greater your astigmatism, the greater this variation of power across the cylindrical lens. The axis number in your prescription provides the orientation of the cylinder power.
Prism Lenses
Prism lenses correct double vision by helping align the two different images that are being input from each eye.
Double vision (diplopia) usually happens when the eyes are misaligned, a condition called strabismus. Prism lens glasses work by redirecting light before it enters the eyes, helping the light center on the retina properly. From there, the brain is better able to combine visual input from each eye to create a single, sharp image.
Prism lenses are not used to correct a refractive error.
When Was Your Last Eye Exam?
A comprehensive eye exam will allow your eye doctor to check the health of your eyes and the clarity of your vision. They’ll also determine what prescription and corrective lenses will work best for your vision needs. Once you have your updated vision prescription, you can shop our wide range of frames for a new pair.
If you haven’t had an eye exam in over a year, now’s the time to schedule your next appointment.
SOURCES
- How the eyes work. National Eye Institute. April 2022.
- Photoreceptors. Kenhub. October 2022.
- Refractive errors. Kellogg Eye Center, University of Michigan Health. Accessed June 2023.
- Eyeglasses for refractive errors. National Eye Institute. July 2019.
- How does a concave lens correct nearsightedness? All About Vision. March 2022.
- Vision correction. College Physics. Accessed June 2023.
- How can presbyopia be corrected? InformedHealth.org. June 2020.
- Cylindrical lens. Academic Accelerator. Accessed June 2023.
- Spherical vs cylindrical lenses| Difference in cylindrical power and spherical power| Astigmatism. EyeCare Advisor (YouTube). April 2021.
- What is prism correction in eyeglasses? American Academy of Ophthalmology. April 2023.
