To put it simply, eyeglasses require prescriptions because a measurable change is needed to improve your vision. The data from the eye exam is evaluated and, based on this information, corrective lenses will be recommended. The actual curvature of the lenses will be a numerical value which we will call the prescription.
How Glasses Work to Improve Vision
To appreciate why eyeglasses require a prescription, a brief understanding of how the eyes work may be helpful. At the back of the eye is the retina, which reacts to light. When you see something, the image is captured as light. The brain translates the light particles into an image that you can identify.
When your vision works perfectly, the light rays focus directly on the surface of the retina. Since the retina is curved, the image needs to be curved. Other parts of the eyes, specifically the pupil and cornea, are responsible for curving the image. If any of these parts of the eye have irregularities, your vision will be blurry.
Nearsightedness, Farsightedness and Astigmatism
Nearsighted people cannot see objects that are at a distance clearly because the light rays focus in front of the retina. The opposite is true for farsighted people because the light focuses behind the retina. A third type of problem is due to an astigmatism, which occurs when there is an irregular curvature of the eye causing a second focal point. In all three of these cases, corrective lenses can improve a person’s ability to see more clearly.
Types of Lenses Used in Eyeglasses
Lenses are curved pieces of glass or plastic that bend the light rays as they approach the eyes. This helps to focus the light on the retina, not behind or before it as with certain vision problems.
There are two main types of lenses: convex and concave. Nearsighted people benefit from convex lenses which curve in slightly and bend the light toward the bottom and top of the lens. This moves the focal point of the light back toward the retina where it belongs. Farsighted people need a concave lens to bend the focal point forward on the retina.
Accuracy of the Prescription
Vision is actually the capture of light on the retina. How well you see is determined by the curvature of the retina and the location of the focal point of the image. Eyeglasses can bend the light to the proper place within the eyes and therefore improve the images that you see.
A prescription is needed for glasses because the curvature of the lenses has a mathematical basis. Too much or too little curve will alter the focal point of the light on the retina and your vision will be affected. A vision exam will measure the precise correction that your eyes need for vision to be improved.
The eye doctor will write out a prescription with the exact type of lenses that are needed. For your vision to improve with glasses, you will need to determine the type of vision problem you have and the mathematical degree of the curvature of the lenses that will treat it. This information is the prescription for your glasses, and is an individual assessment of your eyes detailing the corrective formula.
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