Understanding Vision Problems -- the Basics
Near vision requires focusing or accommodation. The amount of near focusing power decreases throughout life. Presbyopia is blurred vision at normal reading distance. It occurs when the eye develops insufficient focusing power for reading and other near tasks. Presbyopia typically starts at about age 40 and is the reason most older adults rely on reading glasses. Bifocal spectacles permit the wearer to see objects clearly both near and distant.
Visible light rays form images that reach the brain. In order to do that, the retina converts the light signal into a nerve impulse. Think of the retina as silky wallpaper that lines the inside of the eyeball. Unlike wallpaper, however, there is no glue. Small holes can develop in areas where the retina is exceptionally thin or damaged. If that happens, the clear liquid vitreous that fills the eye can seep behind the retina and cause the wallpaper to come off. This is retinal detachment.
Although a detached retina is not painful, it is a medical emergency. If the retina is not reattached to the eye wall promptly, retinal cells starve and permanent blindness can result. Risk factors for retinal detachment include the following:
- Moderate or extreme nearsightedness
- Previous eye surgery or injury
- Previous retinal detachment
- Inherited thinness of retinal tissue
Color blindness is most commonly a disorder of the retina's light-sensitive photoreceptor cells, which respond to different colored light rays. There are two kinds of photoreceptors:
- Cones work best in bright light
- Rods work best in dim light
Each photoreceptor produces pigments that respond to specific colors of light.
Color vision is affected if those pigments are absent or defective or if they respond to the wrong wavelengths.
You have probably seen how paint colors are mixed at the hardware store. Color vision works much the same way because visible light is a mixture of different light rays (wavelengths). Color perception problems occur more often in men, afflicting 8% of the male population. Females tend to be the "carrier" of the trait. It is extremely rare for someone to be totally color-blind, that is, able to see only shades of gray.
Night blindness -- difficulty seeing in dim light -- occurs when rod photoreceptor cells begin to deteriorate. Rods work best in low light. There are many different forms of night blindness, but it may be linked to these conditions:
- Liver disorder
- Vitamin-A deficiency
- Inherited disease of the retina, such as retinitis pigmentosa
Eyestrain can occur simply from overusing your eyes for long periods of time. Eyestrain is discomfort that can also be due to an uncorrected refractive problem. This common vision problem may occur while you are performing distant visual activities like driving or watching a movie or during close-up tasks like reading and computer use.