1. Why defects of vision occur
The human eye works by bending light so that it focuses exactly on the retina. When the eye’s lens system cannot focus the incoming rays properly, the image forms either in front of the retina or behind it, or the eye focuses unevenly in different directions. These focusing errors are called defects of vision.
The good part is that simple lenses can correct most of these defects by helping light reach the retina at the right spot.
2. Myopia (short-sightedness)
Myopia is a condition where near objects are seen clearly, but distant objects look blurry. This happens when the eye focuses light in front of the retina instead of on it.
2.1. Why myopia happens
- The eye’s lens is too strong (overly converging), or
- The eyeball is slightly longer than normal.
Both cases make the rays meet too soon, before reaching the retina.
2.2. How it is corrected
A concave lens is used for correction. A concave lens diverges incoming rays slightly so that the eye’s lens brings them to focus exactly on the retina.
This is why myopic glasses have negative power lenses.
3. Hypermetropia (long-sightedness)
People with hypermetropia can see distant objects clearly, but nearby objects appear blurry. In this case, the eye focuses light behind the retina.
3.1. Why hypermetropia happens
- The eye’s lens is too weak (not converging enough), or
- The eyeball is slightly shorter than normal.
So the rays do not meet by the time they reach the retina.
3.2. How it is corrected
A convex lens is used for correction. A convex lens converges the incoming rays before they enter the eye, helping the eye focus them properly on the retina.
Hypermetropic glasses therefore have positive power lenses.
4. Astigmatism
Astigmatism is a defect where the eye cannot focus light evenly in all directions. Objects may appear distorted or stretched because the eye has different focal lengths in different planes.
4.1. Why astigmatism happens
The most common reason is uneven curvature of the cornea or lens. Instead of being perfectly spherical, it is more curved in one direction than the other.
This makes the eye focus vertical and horizontal lines at different points.
4.2. How it is corrected
A special type of lens called a cylindrical lens is used. It bends light more in one direction than the other, matching the eye’s uneven curvature and restoring clear vision.
6. Combination of defects
It’s possible for someone to have myopia and presbyopia together. In such cases, bifocal or progressive lenses are used to handle near and far vision comfortably.
7. Quick comparison to remember
| Defect | Image forms | Problem | Correction lens | Lens power sign |
|---|---|---|---|---|
| Myopia | In front of retina | Cannot see far objects clearly | Concave lens | Negative |
| Hypermetropia | Behind retina | Cannot see near objects clearly | Convex lens | Positive |
| Astigmatism | Different in different planes | Uneven or distorted vision | Cylindrical lens | Depends on orientation |
| Presbyopia | Behind retina (for near) | Age-related difficulty with near vision | Convex or bifocal lens | Positive for near vision |
8. Why lenses can correct these defects
All these defects arise from incorrect focusing. Lenses fix the problem by adjusting how much light bends before entering the eye. A concave lens spreads rays out, a convex lens brings them closer, and a cylindrical lens fixes uneven focusing.
This simple idea is behind every pair of corrective glasses.