Bifocal contact lenses work similarly to bifocal eyeglasses, providing two different prescriptions in a single lens for close and distant vision clarity. Some bifocal contacts split the lens into two zones, while others have near and far prescriptions arranged in rings and rely on the brain to process visual focus.
Early bifocal contacts used a different prescription in each eye, requiring the wearer to focus with his left or right eye depending on the task at hand. Advancements in contact design allowed manufacturers to combine prescriptions, improving overall vision quality.
Depending on the design, bifocal or multifocal contacts may be oddly shaped, either designed with a flat edge or aspheric shape, similar to contacts designed to correct astigmatism. This keeps the lens from rotating in the eye, ensuring that each focal spot remains where it should be instead of migrating around the field of vision.
Translating bifocals are similar to traditional bifocal glasses, forcing the user to focus his vision upward to look through the distance portion of the lens or downward to focus on close objects. Concentric lenses have a prescription that varies in rings surrounding the center of vision, while progressive contacts change the prescription gradually over the surface of the lens. For these styles of bifocal contacts, the wearer's brain must learn to edit out the portion of the image that is out of focus, relying more on the in-focus zones in the field of vision. This usually takes some time to achieve naturally, and it can cause dizziness, headaches and eyestrain in first-time wearers.