BIFOCAL lenses are simply two lenses with different focal points placed into one single pair of glasses.
PURPOSE OF BIFOCALS:
The eye condition known as PRESBYOPIA is frequently responsible for needing bifocals as a lens treatment program. Essentially, as an individual ages, the natural elasticity of the lens inside the eye diminishes. As a result, the range in focus for the individual is shortened. For example, at the age of 16 the usual range of focus is from infinity up to 10 inches from the face. In contrast, at the age of 70 the range of focus may start at infinity, but stop at 40 inches. Much of our life requires vision closer than 40 inches, therefore, bifocal lenses offer an additional lens with a focal point that will allow clear vision, for that same individual, up to 16 inches.
While presbyopia is frequently responsible for bifocal lenses, there are many other conditions where the bifocal is the best treatment. Some of these are listed below:
Amblyopia (Lazy Eye)
Asthenopia (Eye Strain)
TYPES OF BIFOCALS:
Bifocals are found in many different designs. The most common bifocal used today is found in glasses. It has the shape of the letter "D" laying on its side. These are available in a wide variety of sizes and powers. Other types of bifocals are round, or may be straight across the entire lens. A type of bifocal that has been gaining popularity for several years is the "Progressive" bifocal. Rather than having a distinct line between the two focal points of the lens, the progressive lens has a gradual transition from the top of the lens to the bottom. Doctors may choose this for their patients in an effort to give a broader range of focus as well as diminish the abrupt change when the eyes move up and down over the lens. Your doctor is trained to help you select the best design based on your individual needs. The patients employment activities, recreational needs and reading activities are all considered. While a bifocal for one activity may be perfect, for another it is completely useless. For example, the needs of a "golfer" may be significantly different than an "accountant". Other issues to consider may include the physical charactoristics of the spectacle frames. For example, a frame that is vertically narrow may not allow enough room for a progressive lens to work well.
BIFOCALS & CONTACT LENSES:
Contact lenses also are available in bifocal designs. These come in the form similar to glasses in that there is a dividing line between the top of the contact lens and the bottom. Gas Permiable lenses frequently are in this format. The lens moves up and down on the cornea, depending on the eyelid configuration and the vertical angle of the eye. When this occurs, the lens is said to be "translating" properly. Another type of contact lens bifocal has concentric circles from the center of the lens. These circles alternate between distance vision and near vision. In this way, the eye is looking through two different focal points at the same time. These type lenses are most frequently available in soft lens designs, although some gas permiable materials are available. Additionally, this type of bifocal lens is also available in Intraocular lens implants used at the time of cataract surgery (See: Pseudophakia). The appropriatness of any of these bifocal lens designs should be carefully discussed with your doctor.
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