The eye is a complex organ that works much like a camera, focusing light rays and forming an image. On the surface of the eye is the cornea, a thin, spherical layer of tissue that provides a clear window for light to pass through. In a healthy eye, the cornea bends or refracts light rays so they focus precisely on the retina in the back of the eye.
Beneath the cornea is the iris, the colored part of the eye we refer to when we say a person has brown or blue eyes. In the center of the iris is the pupil. The iris functions like a shutter, adjusting pupil size to control the amount of light entering the eye.
Located behind the iris is the lens, which works together with the cornea and vitreous to focus light. Like the lens in a camera, it adjusts light rays as vision shifts between nearby and distant objects in a process called accommodation.
Light then passes through the vitreous, the gelatinous substance that fills most of the eye and gives it its shape.
The back of the eye is lined with a thin layer of tissue containing millions of photoreceptor (light-sensitive) cells. This is the retina, where light rays focus into an upside-down image. In the center of the retina is the macula. Less than 1/4 of an inch in diameter, the macula is responsible for clear central vision. The retina converts the image into an electrical signal that travels down the optic nerve to the brain.
Nearsightedness, or myopia, is a vision condition in which nearby objects are clear and distant objects appear blurry. This may be caused by excess corneal curvature or an oblong rather than a spherical shape to the eye, both of which affect the way light is bent upon entering the eye and whether it focuses properly on the retina.
Almost a third of people in the U.S. experience some degree of nearsightedness, which normally emerges by age 20. Symptoms include difficulty focusing on objects in the distance, such as a chalkboard or TV. There is some evidence that it is caused or worsened by sustained focus on nearby objects. Nearsightedness may also be hereditary.
Eyeglasses and contact lenses are common methods of correcting nearsightness. Eyewear may be used for certain activities, like watching television or driving, or for all activities. Alternately, vision correction procedures such as refractive and laser surgery and may rectify the problem.
Farsightedness, or hyperopia, is the condition in which the eye focuses on distant objects better than on objects closer to the eye, so nearby objects appear blurry. This happens when light rays refract, or bend, incorrectly in the eye. The eye is designed to focus images directly on the surface of the retina; when the cornea is incorrectly curved, light rays focus behind the surface of the retina, producing a blurry image.
Hyperopia can be treated in a variety of ways. The most common is a pair of reading glasses or contact lenses. Hyperopia can also be treated with invasive or non-invasive medical procedures, including the laser surgeries PRK (photorefractive keratectomy) and LASIK (laser-assisted in situ keratomileusis).
Astigmatism occurs when the cornea - the clear covering over your eye that bends (refracts) light - is slightly irregular in shape, preventing light from focusing properly on the retina in the back of your eye. As a result your vision may be blurry at any or all distances.
Astigmatism can cause symptoms of varying severity, from headaches, eye strain and fatigue to blurred or distorted vision.
Most people have some degree of astigmatism. Eye exams test for the condition and help your optometrist provide a proper prescription for eyeglasses or contact lenses. Depending on lifestyle and the degree of astigmatism, some patients prefer corneal modification (i.e. LASIK or other refractive surgery) to improve vision quality.
Presbyopia is a natural change in our eyes' ability to focus. It occurs when the soft crystalline lens of the eye starts to harden. This loss of flexibility affects the lens' ability to focus light in the eye, causing nearby objects to look blurry. Presbyopia happens to everyone starting in about our 40s or 50s -- even in patients who have had laser vision correction.
The effects of presbyopia can be corrected with glasses or contact lenses, including bifocals and multifocals; multifocal lens implantation, including Crystalens™, ReZoom® and ReSTOR®; conventional surgery; and monovision LASIK. Laser surgeries such as conventional LASIK, PRK, and Custom LASIK cannot correct presbyopia because they reshape the cornea rather than treat the lens.