Multifocal glasses are eyewear designed with two or more corrective powers within a single lens. This allows the wearer to see clearly at multiple distances without needing to switch glasses. People require this technology primarily due to presbyopia, a natural, age-related decline in the eye’s ability to focus on close objects. This hardening of the eye’s crystalline lens begins around the early to mid-forties, making activities like reading small print increasingly difficult.
Understanding the Design of Multifocal Lenses
The concept behind a multifocal lens is the strategic placement of different prescription strengths across the vertical plane of the lens surface. Unlike a single-vision lens, which contains only one power, a multifocal lens creates a gradient of optical correction. For clear distance vision, such as driving, the wearer looks through the upper portion of the lens. The lower segment contains the strongest added power, optimized for near-vision tasks like reading. The space between these two extremes addresses intermediate distances, such as viewing a computer monitor or a car dashboard.
Differentiating Bifocals, Trifocals, and Progressives
Multifocal correction is achieved through three main designs, each offering a distinct visual experience and number of focal zones.
Bifocals
Bifocal lenses were the original solution, containing two distinct power zones: distance vision in the upper portion and near vision in a smaller segment at the bottom. These two zones are separated by a noticeable, visible line that runs horizontally across the lens. The primary drawback of this design is the phenomenon known as “image jump,” which occurs when the wearer’s line of sight crosses the dividing line. Furthermore, bifocals offer no correction for objects at an arm’s length, a common viewing distance in modern life.
Trifocals
Trifocal lenses are an advancement of the bifocal design, incorporating a third power zone to address the uncorrected intermediate distance. They feature two distinct horizontal lines, dividing the lens into three sections: distance vision at the top, a narrow intermediate corridor in the middle, and the near-vision segment at the bottom. The intermediate zone is useful for tasks like reading a computer screen or viewing objects about 18 to 24 inches away. Although trifocals solve the issue of intermediate vision, they retain visible dividing lines, which can be aesthetically unappealing and cause image shift.
Progressive Lenses (PALs)
Progressive Addition Lenses (PALs) are the most modern and common type of multifocal, distinguished by the absence of dividing lines. Instead of distinct segments, the lens surface provides a seamless, gradual change in power from the distance prescription at the top to the reading power at the bottom. This smooth transition eliminates the image jump associated with lined multifocals. The trade-off for this natural, cosmetically appealing vision is the presence of peripheral distortion, or “soft blur,” along the edges of the lens.
Tips for Successful Adaptation
Adapting to multifocal lenses, particularly the progressive design, requires a period of adjustment for the eyes and the brain. This adjustment period varies by individual but lasts between a few days and up to two weeks. Consistent, full-time wear is the most important factor for reducing this time and allowing the brain to map the different power zones.
To use multifocals effectively, a change in head and eye movement is required. Instead of moving the eyes side-to-side, a wearer should move their head to “point their nose” at the target to look through the clearest central part of the lens. For reading, the chin should be tucked slightly to look down through the lower, strongest-powered segment.
Initial symptoms like mild headaches, eye strain, or temporary dizziness are common as the brain processes the new visual information. Wearers must be cautious when walking down stairs or stepping off curbs, as looking through the lower reading zone can distort depth perception. If discomfort or blurring persists beyond the two to three-week mark, a consultation with an optometrist is necessary.