Vision Problems: Which one do you have? What does it mean?

A normal eye works when rays of light pass through the lens and cornea at the front of the eye, are bent or refracted, and come to a focal point on the macula, a part of the retina at the back of the eye.

When the lens and cornea are shaped improperly, light is refracted differently and is focused either behind the retina or in front of it.

Myopia (Nearsightedness)

When the eyeball is too long or the lens system has too much focusing power, light rays come to a focal point in front of the retina.  This refractive error is called myopia or nearsightedness.  People with myopia can see images that are close, but have a problem focusing on objects at a distance.

Hyperopia (farsightedness)

When the eyeball is too short or the focusing power of the lens is too weak, light is focused behind the retina.  This error, called hyperopia or farsightedness, causes near vision to be blurry.

Astigmatism

Astigmatism is a distortion in the shape of the cornea.  In eyes with this error, light is not focused into a single point.  Instead, the light rays entering the eye at the top and bottom form a different line from the light entering at the sides.

Presbyopia

As eyes get older, the lens of the eye becomes less flexible, and the eye has a harder time focusing images that are up close.  This occurrence, called presbyopia, is a part of the normal aging progress and happens to everyone, usually by the age of 50.

Terms Associated with Vision Problems

Diopter

Diopter is the measurement of refractive error: the larger the number, the greater the refractive problem.  On a vision prescription there are spaces for four different numbers reading from left to right.  The numbers indicate the amount of refractive error (measured in negative diopters for nearsightedness and positive diopters for farsightedness), amount of astigmatism (measured in diopters), the axis of the astigmatism (the place at which the cornea is either the steepest or flattest), and the amount of near vision correction (indicating the need of bifocals for correction of presbyopia).  A nearsighted patient with -1 D can see objects clearly at 1 meter, a patient with a -2 D error can see objects clearly at 1/2 meter.

Visual Acuity

Visual acuity refers to the sharpness or clarity of vision.  Acuity is measured in the United States in comparison to what a normal eye, without refractive errors, can see at a distance of 20 feet, so that a normal eye is measured at 20/20.  A person with 20/100 vision can see at 20 feet what a normal eye can see at 100 feet, a person with 20/40 vision can see at 20 feet what a normal eye can see at 40 feet, and so on.  In some cases, people have vision better than 20/20.  For example, a person with 20/15 vision can see at 20 feet what a normal eye would have to be 5 feet closer to see.

Correcting Refractive Errors

The most common corrections for myopia, hyperopia, and astigmatisms have been glasses or contact lenses, but more and more patients are opting for surgical corrections includingLASIK, LASEK, PRK, and phakic IOLs.  Presbyopic patients have generally had to rely on reading glasses or bifocals, but recent advances, including CK and multifocal IOLs, have allowed a surgical option for the correction of presbyopia. 

source: http://www.locateadoc.com