Prescription eyeglasses are glasses that have been prescribed by an optometrist to be worn by someone who has been diagnosed with a certain eye condition. Depending on the condition, a different lens will be prescribed. Prescription eyeglasses are broken down into four different categories. The Single Vision Lens has the same focal power from the top of the lens to the bottom of the lens. People who have been diagnosed as farsighted, nearsighted or with having astigmatism can wear single vision lenses to correct these types of eye disorders. Astigmatism is a condition that doesn't allow light to reflect on the retina, thus causing distorted images. A Bifocal Lens is used to correct a condition call Presbyopia. This disorder usually occurs in people in their late 40's. If a person has been diagnosed with Presbyopia, they can't focus in on close objects. That is where the Bifocal Lens is helpful. The top of the lens is used for distant vision and the bottom of the lens is helpful with close vision. People with Presbyopia need both types of Focal View on their prescription eyeglasses. The Trifocal Lens-meaning they have three focal views-is designed similar to the bifocal lens with the exception to the center focal point on the lens is for intermediate vision. The fourth type of lens is the Multi Focal lens. It has more than one focal view; therefore a Bifocal and a Trifocal are types of multi focal lenses.
Other types of lenses that are specially designed for other purposes include the Computer Lens. They are designed so that when sitting at a computer, a person can view the monitor and papers nest to the monitor at the same time. And then there are Double Bifocals, which are rarely prescribed, but when they are it is usually for pilots, electricians or other professions that need to have close vision overhead.
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