What Are Concentric Rings on Bifocal Contact Lenses?
Bifocal contact lenses correct presbyopia, a vision condition that happens to most people between ages 40 and 45. Smaller details become more difficult to see. This happens because the eye's crystalline lens becomes less elastic and does not work as well. Bifocal contact lenses combine both far and near prescriptions to correct for presbyopia. These types of lenses work in several ways, one of which uses concentric rings.
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Types
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Aspheric and concentric ring are the two most common types of bifocal contacts. Although individual prescriptions and fit determine which works best, a general guideline is that aspheric bifocal lenses work better for lower power (less strong) prescriptions.
Aspheric lenses have both distance and near prescriptions in the center of the lens. Concentric ring bifocal contact lens have a prescription in the center and one or more rings of power surrounding it. Also referred to as an annular lens or simultaneous vision, this type of lens has the near correction and the distance correction in layers of rings within the contact.
A third common type of bifocal contact fitting is monovision. Patients are fitted with one lens for far vision and one lens for near vision, and the brain determines which eye to use at various distances. These lenses are not true bifocal lenses, but rather lenses with very different powers.
Material
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Concentric ring bifocal contacts can be made of gas pPermeable (RGP) or soft material. Rigid lenses usually have the distance correction in the middle of the lens, called center-distance. Soft bifocal contacts with concentric ring design usually have the near correction in the center and are called center-near.
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Significance
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The simultaneous vision of concentric ring bifocal contact lenses helps avoid "image jump," a common problem associated with other types of lined bifocals. Image jump happens when the eye looks through the lens, crossing the lined bifocal. Power from the distance jumps to the near, causing a feeling of disruption. The simultaneous vision of concentric ring contacts works well for people who are bothered by image jump.
Effects
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Because the distance and near powers of the rings are in the line of sight at all times, the eye and the brain must play catch-up with each other. As the eye looks through the contact, light from distant objects can pass through the near rings, and light from close objects can pass through the far rings; the eyes are receiving information for objects that are both in and out of focus all at once. It may take awhile to adjust to this vision, and not everyone likes this type of correction.
Considerations
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Finding the best type of bifocal contact lens can involve some trial and error. Most eye doctors will give you a trial fit pair of lenses to wear for a week or so and then return for another appointment. Sometimes, several trials are needed to find the best type of bifocal lens fitting.
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