The human eye naturally loses the ability to focus on close objects as a person ages, a condition known as presbyopia. This loss of flexibility typically begins around age 40, leading to blurred vision for reading and other near tasks. For those who already wear contact lenses or prefer not to wear spectacles, the need for reading glasses can be inconvenient. To address this, manufacturers developed lenses that incorporate multiple prescriptions within a single lens, widely known as multifocal or progressive contact lenses. These lenses are designed to provide clear vision at far, intermediate, and near distances, offering a seamless visual experience without the constant need to switch eyewear.
Understanding Progressive Contact Lens Design
Progressive contact lenses function based on the principle of simultaneous vision. This means the lens presents multiple powers to the retina at the same time. Unlike standard single-vision lenses, multifocal lenses contain distinct optical zones with different refractive powers. Light rays from distant and near objects pass through the lens simultaneously, creating two images on the retina—one clear and one slightly blurred.
The visual system, specifically the brain, learns to selectively attend to the clear image corresponding to the object of focus and ignore the blurred one. This process requires a period of “neuro-adaptation” as the brain trains itself to filter the incoming visual information. The arrangement of these optical zones often transitions gradually in power across the lens surface, ensuring that the appropriate power is always positioned in front of the pupil, whether the wearer is looking far away or reading a book.
Different Types of Multifocal Lenses
While all progressive contacts employ the simultaneous vision strategy, they utilize distinct structural approaches to arrange the various power zones across the lens surface. The two primary designs are concentric and aspheric, each offering a different visual experience.
Concentric Design
Concentric multifocal lenses feature a bullseye pattern, consisting of alternating rings of distance and near prescription that extend outward from the center. These designs may be center-distance or center-near. The distinct zones often provide sharp vision but can sometimes cause a noticeable visual jump or halos, especially in low light.
Aspheric Design
Aspheric multifocal lenses, conversely, feature a more blended and progressive design, similar to progressive eyeglass lenses. In this design, the prescription power gradually changes from the center of the lens to the edge without distinct rings or segments. This smooth transition often results in a more natural and comfortable visual experience, minimizing the visual disturbances sometimes associated with concentric rings. However, this blending of power can sometimes lead to slightly less sharp vision at the extreme near or far points compared to the more defined zones of concentric lenses.
The Fitting and Adaptation Experience
Obtaining progressive contact lenses requires a specialized fitting process. This begins with a comprehensive eye examination to determine the precise distance prescription and the necessary near-vision addition (add power). The eye care professional must also consider factors like the patient’s pupil size, which affects how the lens design functions, and their overall visual demands.
The fitting involves a trial period where the patient wears a set of lenses to evaluate their performance in real-world conditions. Because different manufacturers use unique optical designs, the success of the fit relies heavily on the practitioner’s expertise in selecting the most suitable lens type for the individual’s eyes and lifestyle. The cost for this specialized service is typically higher than for a standard contact lens fitting.
A significant component of the process is the adaptation period, during which the brain learns to process the multiple images presented by the lenses. This neuro-adaptation can take up to two weeks, and patients are advised to wear the lenses consistently during this time to expedite the adjustment. Patients should maintain realistic expectations, as multifocal lenses may sometimes cause a slight reduction in contrast sensitivity or minor issues with glare at night. Patient motivation is necessary for long-term success, as the willingness to complete the adaptation period is crucial.