Potential problems in the optical stage – refraction
When light rays do not converge on the retina (See >>>LINK) precisely, several types of vision defects may occur.
The ability of the eye to focus properly depends on a few aspects of the eye:
- The cornea’s curvature
If the front surface of the eye, the cornea, is not adequately spherical, if its the curve is irregular, light rays entering the eye will not focus correctly on the retina, resulting in a blurred image. This is referred to as astigmatism.
- The length of the eye
If the eye’s shape or length prevents the focused light from landing on the retina, vision will be distorted.If the eye is too long front to back, light that is focused in front of the retina. This is Myopia or nearsightedness –you can see better up close than far away. Myopia can be corrected with concave lenses.
- If the eyeball is too short, the light that is focused in the eye lands behind of the retina and one is farsighted. This is called Hypermetropia, which can be corrected with concave lenses, which increase the light rays’ convergence, pulling the focus back onto the retina.
- The curvature of the lens inside the eye
Nearsightedness and farsightedness can also result from distortion of the lens’s curve. A too-flat lens will result in farsightedness, a too-curved lens in nearsightedness.
As well, the lens loses its natural elasticity with age losing its ability to change shape, making it more difficult to focus on close objects, as we do when reading or sewing or typing on a keyboard. This hardening of the lens is called Presbyopia, which can be compensated for with reading glasses. In fact, by the age of 30 you have lost around half of your accomodative power.
Vision can be diminished by corneal opacities that can lead to scarring or clouding of the cornea. Corneal opacities can cause minor irritation to severe vision problems. Corneal opacities can also result from bactrial infections from severe Conjunctivitis and contact lens-related infection.
A cataract is a clouding of the eye’s lens that occurs when some of the protein that keeps the lens clear and lets light pass through it clumps together and starts to cloud a small area of the lens. Mostly related to aging, the clouding affects vision and often results in surgery to remove the cataract.
Although cataracts are related to aging – more than half of all Americans 80 and over either have or have had a cataract – they can also result from smoking and diabetes.