Eyeglasses are such a common sight amongst people that it is often not even given a second thought. In fact, about 160 million people in the United States alone use corrective lenses to improve their vision. However, understanding why so many people need corrective lenses begins with the basic science of how eyesight works.
How Eyesight Works
In the most simplistic description, your eye is similar to a camera. It has a variable opening called the pupil, which is like a camera lens. You eye also includes a transparent covering called the cornea, a curved lens system, and a retina, which acts like reusable “film”. The eye contains various muscles that control the size of the pupil, the shape of its lens system, and eye movements. The complex set of muscle movements in the eye is controlled automatically by your nervous system.
Light passes through the cornea and pupil and is bent or refracted by the lens. It then comes to a point of focus on the retina at the back of the eye. Ability to focus light on the retina depends on the shapes of the cornea and lens in the eye. This is controlled by their inherent shapes, elasticity, and the sets of attached muscles. The layer of sensory cells called rods and cones in the retina then reacts to light, and the reaction is sent to the brain, which forms the image that you see.
If you have perfect vision, the light rays focus directly on the surface of the retina. The formed image is shrunk and curved because the retina is curved. The pupil and cornea are in charge of shrinking, focusing, and curving the image. Otherwise, if there are any irregularities, your vision will be blurry and and in need of corrective lenses.
Here are the most common reasons for blurry vision:
- A person can see close-up objects well, but distant objects are blurry
- Occurs because the light from distant objects gets focused in front of the retina rather than on it
- Usually caused by the eyeball is too long or by too much focusing power in the eye lens system
- Corrected with a concave lens to spread out, or diverge, the light when it passes through the lens system, so it can come to focus on the retina
- The shape of the eye causes the light rays to come into focus behind the retina, which results in seeing distant objects fine, but close-up objects are blurry
- Usually occurs when the eyeball is too short or by weak focusing power of the lens system
- Can be corrected with a convex lens to concentrate, or converge, the light so it focuses on the retina when it passes through the lens system
- Frequently develops as people age
- Blurry vision caused by a distorted curvature in the eye, which results in light entering second focal point
- The eye lens system is egg-shaped rather than spherical, so light through the top and bottom edges is brought to a different focal point than light through the right and left sides
- Corrected with a cylinder curve lens that is shaped to fix the distorted shape of the eye’s lens system
- Trouble focusing light from both near and far objects on the retina
- The cornea and lens of the eye are less stretchy and cannot change shape as readily to bring light into focus on the retina
- Happens naturally as we grow older, specifically when a person is 40 and older.
- Correct this problem with a pair of bifocal lenses, where the top part is for seeing far objects and the bottom part is for seeing near objects