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What is 20/20 vision?
It is a measurement of sight in high contrast (black against white) conditions. If you can see a letter 8.7mm tall at 20 feet than you have 20/20 vision. If you have to be closer- say 10 feet away - to see this same object than you have 20/40 vision. The Snellen Test, which measures 20/20, does NOT mean you have perfect vision since it is only a measurement of one facet of vision. It is a decidedly low tech test which was developed by a Dutch ophthalmologist in 1862. In fact many people with 20/20 still require eyeglasses to give them more efficient vision.
Amazingly, it is actually possible to be legally blind and have 20/20 vision!
One of the two definitions of legal blindness depends on your visual field (or peripheral vision) and if it is less than 20 degrees then you are legally blind. Either visual acuity of less than 20/200 in the better eye with the best correction or a visual field of less than 20 degrees is the definition of legal blindness. For all practical purposes, with the limited visual field, you would be unable to function independently in a normal world. You would always be running into things and bumping into them. Your vision without glasses or contact lenses does not determine legal blindness - only your best corrected vision with glasses or contact lenses.
What are cataracts?
Cloudiness of the crystalline lens of the eye. A clouded lens interferes with light reaching the retina, causing blurred vision. Often times your vision on a Snellen Acuity Chart (20/20) will be better than it actually is in real life. This is because your contrast sensitivity will be greatly diminished and your sensitivity to glare will be worsened. Contrast sensitivity refers to your ability to see in reduced contrast situations such as when lighting conditions are less than optimal. Cataracts rarely lead to blindness because they can be corrected by surgery. After eye surgery you may not even be required to wear eyeglasses. Some of the newer lens implants are "multifocal" which means they can take the place of bifocals.
What is Astigmatism?
A visual defect caused by an eye surface that is irregularly shaped. People with astigmatism see less clearly at any distance. Astigmatism will cause a different focus in different directions. Many people think that astigmatism is a serious eye problem, but it can be easily corrected by glasses or contact lenses or LASIK.
What is nearsightedness?
It is also called myopia. You can see clearly up close, but not at a distance. Myopia tends to become progressively worse throughout childhood especially for children who do a lot of reading and close work and who don't spend much time outdoors. Myopia levels off until around age 40 or before. Myopia oftentimes is found together with astigmatism.
What is farsightedness?
It is also called hyperopia. It means that you have to strain your eyes to focus. If you are young you may still be able to see clearly at both far and near. If you are over 40 you may not see well at either far or near. If you are at an in-between age, you will see better at far than near, but still not well. That may sound confusing because it is not the opposite of nearsightedness in its effects on your vision.
What is Presbyopia?
Presbyopia is actually the most common focus problem and yet is a term most people have never heard of. It literally means "old eyes" and is therefore universal since everyone eventually gets older - if they make it that far. The most common age where it begins to affect your vision is at the age 43. It causes you to have to hold objects farthur away from your eyes in order to see clearly than when you were younger. It can be corrected by glasses, contacts, or LASIK.
What is LASIK?
LASIK is a refractive surgery procedure that can eliminate the need for glasses or contact lenses for many people. On the other hand, there are many people for whom it is not an ideal solution. It does not correct eye conditions that do not correct with glasses or contact lenses and is not permanent. However, many people who have had it done are thrilled with the results.