The eye has a built-in focusing system which can increase its focusing power, and can return the focus back to start. The school age child can see the board clearly, then look at a book and see it clearly, then refocus back to the board. This change in focus is usually so natural that the person is not even aware that his eyes are changing focus. The closer an object is the more power it takes to focus in.
Alas, this system doesn’t last forever the ability to change focus gradually diminishes over time so that by the mid 40’s the remaining focusing power has declined so much that reading has become difficult; and by the age of about 55 years there is no ability to change focus left. This loss of focusing power is called presbyopia. It is the reason that people find themselves moving their book further away when they get to their 40’s. It is the normal way that the eyes change with time and is experienced by everyone.
Benjamin Franklin, one of America’s founders, invented the bifocal. This is an eyeglass lens that focuses the long distance when looking through the top, and the close range when looking through the bottom. The bifocal lens compensates for presbyopia.
Bifocal lenses of the 20th century and earlier had a variety of lines on them which separated different zones of focus. Now multifocal lenses may or may not have lines. Progressive addition lenses have no lines. The top section of the lens focuses far away. As your eye looks gradually lower down the lens, gradually closer distances come into focus. In order for progressive addition lenses to work properly it is essential that they are positioned properly in your eyeglasses and in front of your eyes. This depends on the skill of the person helping you select an eyeglass frame and making the proper measurements. In addition, your eye care practitioner should help you select the correct type of lens for your individual needs. There are many ways that contact lenses can be used to correct presbyopia.
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Posted In: Presbyopia