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Bifocals, Multifocals y Aspherics

Presbyopia
As we get older our eyes lose the ability to change focus from distant objects to near ones. This loss of accommodation is called Presbyopia, and it happens to just about everyone, usually starting around age 40. For most people, the biggest problem with Presbyopia is reading. As the ability to focus on close objects gradually diminishes, reading becomes more difficult, particularly when the print is small, the lighting poor, or the reader is tired.

Good News, Bad News
Presbyopia is an entirely natural and normal phenomenon. While Presbyopia won't go away, our eyes won't learn to accommodate again, there are many simple and effective ways to deal with it. In the past, reading glasses or bifocals took care of the problem for just about everybody. Today soft and rigid contact lenses can also provide visual correction for people with Presbyopia.
That's the good news.

The bad news is that contact lens correction of Presbyopia is not for everybody. While many wearers are quite pleased with their bifocal contact lenses, others find themselves either unable to adjust to the lenses or dissatisfied with the vision they provide. In addition, bifocal contact lenses usually cost more than standard "single vision" lenses. Your vision (without lenses) and your specific near visual needs are important in determining whether bifocal contact lenses make sense for you. Presbyopes (people with Presbyopia) often need correction for both distance and near vision. Presbyopes with correction for near vision only are the worst candidates for bifocal contact lenses. So, if you are using reading glasses only and you can see clear at distance without any correction, your chances for this contact lenses are not in your favor.


Correcting Presbyopia with Segmented Lenses

Bifocal glasses have a distance portion and a reading (near) portion. You look through the upper (distance) portion of the lens to see far objects and through the lower (near) portion of the lens when reading. Some bifocal contact lenses are designed in a similar way. Like bifocal spectacle lenses, contact lenses can be made with the distance portion on top and a near portion at the base. Bifocal contact lenses designed this way are called segmented lenses. While the basic design of segmented contact lenses reminds one of bifocal glasses, segmented contact lenses don't work in exactly the same way. With a bifocal spectacle lens in front of your eye you can direct your gaze through the portion of the lens you need. But this won't work with a contact lens that's on your eye. If you are wearing a segmented contact lens and your gaze shifts from far to near, the lens itself must move slightly to bring the near-vision segment into your line of sight. Of course, the lens will have to move back again when you want to see objects at a distance. A number of design features help to position the segmented lens on the eye. For example, the lens may have a flat bottom edge to allow the lower eyelid to assist in positioning; or it may be weighted on the bottom to prevent rotation. Because lens position is critical, the fitting of segmented lenses requires skill and patience. Several visits to the contact lens practitioner may be required in order to get a fit that's just right. Segmented contact lenses provide alternating vision. That is, when you look through the near-vision segment, close objects are in focus; and distant objects are in focus when you look through the distance segment.

[Segment Description]

When you look through the near-vision segment, the lens will rides up pushed by the lower lid; When you look through the distance segment lens return to the normal position. 

Alternating Bifocal Contact Lenses

[Panosite bifocal][Panosite Bifocal]


Simultaneous Vision

A different kind of bifocal contact lens allows both distant and near objects to be in focus at the same time. Called simultaneous vision, this design puts the near correction and the distance correction in concentric rings. Since both the distance and near portions of the lens are within the line of sight at all times, light from both distant and near objects can be in focus in the eye at the same time. But this has limitations. While the eye is seeing through the contact lens, some light from distant objects is passing through the near-vision part of the lens and some light from near objects is passing through the distance portion. So the eye receives both in-focus and out-of- focus images at the same time. In simultaneous vision, it is up to the brain to select the desired image. Simultaneous vision lenses allow for a simpler lens design, the lens position is not as critical as it is for segmented designs, but not all wearers are able to adjust to simultaneous vision.

[Aspheric Multifocal]


Aspheric Contact Lenses

In addition to the standard simultaneous vision bifocal contact lenses that put the distance and near corrections in concentric rings, there are several related lens types. Aspheric contact lenses change lens power gradually, from the center to the edge of the lens. Because the change in lens power is gradual, correction for intermediate distances is possible. The diffractive contact lens uses a series of concentric grooves, cut in the back surface of the lens, to provide the near-vision correction. In general, image resolution is better with diffractive lenses than with other simultaneous vision lenses, and there is less initial blur. Occasionally in dim light, some wearers report seeing a "ghost" image or halo effect in certain lighting situations

[Simultaneo Multifocal] The brain cleans and adjust the images as the it is needed.

Cost of Bifocal Contact Lenses

Bifocal contact lenses usually cost more than single-vision lenses because:

Fitting bifocal lenses is more complex. It takes more time and will likely require more visits before a satisfactory fit is achieved.
The lenses themselves, being more complex, cost more.

Getting Used to Bifocal Lenses

Adapting to bifocal contact lenses takes time in addition to the normal time it takes to adapt to any other contact lens. If you would like to wear bifocal contacts, you should be prepared for a period (likely to be several weeks) of adaptation. In addition, you should be prepared for the possibility of extra visits to the contact lens practitioner to ensure that the lenses fit properly and meet your needs.
Presbyopes should also be prepared for the possibility of failure. It may turn out, for example, that simultaneous-vision lenses just aren't for you. In that case your contact lens practitioner may suggest some other option, such as monovision or segmented lenses. A strong desire to wear contact lenses is a big help here. It can take a while (and sometimes one or two failures) to find a solution that meets your needs. If you're unsure whether you really want to wear contact lenses, the time and expense required to get a satisfactory fit with bifocal lenses may prove too much.

Final Word

While it may take some time for Presbyopes to get used to bifocal contact lenses, once fit with a pair of contact lenses that work, they're just like any other contact lenses. After the initial fitting period, you won't have to return to the doctor more often than other contact lens wearers. And, because your lenses are made of the same high quality materials as other contact lenses, they require the same care. Though the initial fitting is more involved, once you're happy with your lenses, you're likely to stay that way.


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