- Products & Services
- Knowledge Base
SAN FRANCISCO, CA, March 06, 2009 /24-7PressRelease/ -- Have you ever wondered how your eyes actually work? There are many delicate parts to the eye that work together almost like a camera, including the:
• Pupil, the "black dot" in the center of the eye
• Cornea, the clear covering of the eye
• Crystalline lens, behind the iris
• Ciliary muscles, controlling the lens curvature, known as accommodation (ability to focus at all distances)
• Retina, a light-sensitive membrane covering the back of the eye
• Optic nerve, running from the retina to the brain
How the Eye Works
Light passes through the cornea and the pupil, then through the lens. The image is focused on the retina, where sensory cells known as rods and cones translate the image to electrical energy and pass it to the tiny optic nerve fibers in the retina. They converge in a large nerve which exits the back of the eye and carries the image data to the brain to be interpreted.
The cornea is clear and curved and acts like a lens. It bends (refracts) light as it enters the eye. The lens is also clear and curved and refracts the light further. In a 20/20 eye, their combined light refraction focuses the light rays on the retina.
When light travels from a distant object it needs less refraction than light coming from a close-by object. The ability of the lens to change its curvature "accommodates" that by bending incoming light at a greater or lesser angle. The corneal curvature is fixed.
Four Types of Refractive Error
Your eyes have a refractive error if:
• Book print looks blurry (you are farsighted or presbyopic)
• You cannot recognize a person standing a block away (you are myopic)
• Things look a bit blurry at all distances (you are astigmatic)
In those scenarios your eyes are not refracting (bending) light well enough to give you clear vision at all distances.
1. Nearsightedness (myopia)
A myopic eye refracts light too much. The cornea and the lens between them are focusing light before it gets to the retina. So when it does get to the retina it has diverged again and gives you blurry vision. However, it is only relatively distant objects which look blurry while the book print is clear.
That is because light from close objects needs to be refracted more than light from distant objects. So your eyes are doing well for close objects and need to be corrected for far objects.
2. Farsightedness (hyperopia)
A hyperopic eye refracts light too little. The light is focusing behind the retina (except that light cannot travel through this dense tissue, so in effect it is not being focused at all within the eyeball. This is fine for light coming from far objects, since it needs less refracting. This light does focus on the retina.
Light from near objects needs to be more sharply refracted. But the hyperopic eye cannot do that so these images look blurry.
An astigmatic eye has an oval-shaped cornea. It has a steeper curvature on its shorter side and a flatter curvature on its longer side. It refracts light in two ways, causing some light to focus on the retina and some to focus in front of or behind the retina. This gives blurry vision at all distances. You can be both astigmatic and myopic or astigmatic and hyperopic.
Presbyopia is age-related. Everyone starts to become presbyopic after the age of about 40 or 50. You may have 20/20 eyes and still be presbyopic, which means that book print is blurry while intermediate and distant objects are clear.
Causes of presbyopia are not fully understood, but it is related to the lens being less able to change its curvature. It becomes fixed within a narrower range of curvatures which maintain clear distance vision but progressively give blurry vision at closer distances. Subjectively it is like being farsighted but the cause is different.
Diagnosing Refractive Errors
Any eye doctor can test your vision to determine your eyes' refractive errors. For glasses or contact lenses, an instrument called a phoropter is used. It is a device that positions a variety of different corrective lenses in front of your eyes. You read an eye chart while looking through the lenses, and the eye doctor will switch between different lenses until you can see the eye chart clearly.
Custom LASIK uses a more microscopically accurate method of diagnosing the eyes' refractive errors. It is called Wavefront technology.
If your eyes are in good general health, refractive errors are nothing to fear. Today's advanced vision technology offers solutions for clearer vision, including LASIK and its variations.
In San Francisco and surrounding areas, The Laser Eye Center of Silicon Valley can help you see more clearly.
Source: The Laser Eye Center of Silicon Valley
# # #