After the age of 40, many people experience presbyopia, the gradual loss of the eye’s ability to focus on nearby objects. Fortunately, a single pair of glasses can now address this change, allowing clear vision for both far-off objects and close-up tasks. Modern lens technology, often referred to as multifocal lenses, provides a seamless way to correct distance, intermediate, and near vision within one lens. This eliminates the inconvenience of constantly switching between separate pairs of glasses.
Understanding Multifocal Lenses
Multifocal lenses are designed to contain multiple optical powers to correct vision across various distances. The earliest forms, bifocals, feature two distinct power zones separated by a visible line on the lens surface. The upper segment is typically for distance viewing, while the lower segment provides the necessary magnification for reading and close work. A limitation of bifocals is the lack of correction for objects in the arm’s-length or intermediate zone, such as a computer screen.
Trifocal lenses expand on this concept by adding a third segment for intermediate vision, situated directly above the reading portion. Trifocals also have visible lines that delineate the three distinct zones: distance, intermediate, and near. Both bifocals and trifocals suffer from “image jump,” an abrupt shift in focus that occurs when the eye crosses the visible line separating the power segments.
Progressive Addition Lenses (PALs) offer a contemporary solution by providing a smooth, continuous transition between all focal points without any visible lines. The lens is structured with the distance prescription at the top, a reading area with the full near power at the bottom, and a “progressive corridor” in between. This corridor is the area where the lens power gradually changes, accommodating the intermediate viewing range. This design provides a more natural and continuous field of vision.
The Process of Obtaining Your Lenses
Obtaining multifocal lenses begins with a comprehensive eye examination to determine the necessary prescriptions for all viewing distances. This exam establishes the power required for distance vision and the “add power,” which is the additional magnification needed for near tasks. The precision of these prescriptions is foundational for ensuring the correct lens design and power distribution.
Once the frame is selected, the optician must take measurements to ensure the lens functions correctly for the wearer. Pupillary Distance (PD), the measurement between the centers of your pupils, must be measured for each eye individually (monocular PD). The optical center of the lens must align exactly with the center of the pupil for clear vision.
For progressive lenses, the fitting height, or “OC height,” is the vertical distance from the bottom of the lens frame to the center of the pupil when the wearer is looking straight ahead. This measurement determines where the progressive corridor and the near-vision zone will be positioned within the lens. If the fitting height is incorrect, the wearer may struggle to access the reading zone or experience distortion.
The physical characteristics of the chosen frame also play a role in the success of multifocal lenses. Progressive lenses require a certain amount of vertical space to accommodate the seamless corridor of power change. If a frame is too shallow, the near-vision portion may be cut off, limiting the effectiveness of the lens for reading. Opticians often recommend frames with sufficient vertical depth to ensure the entire range of vision zones is available to the wearer.
Adapting to Dual-Vision Correction
The transition to wearing multifocal lenses, particularly progressives, requires an adjustment period. This adaptation typically takes anywhere from a few days to a couple of weeks of consistent wear. The most common initial sensation is a feeling of blurriness or distortion in the peripheral areas of the lens, often described as a “swimming” effect.
This peripheral distortion occurs because the power change along the progressive corridor creates areas of unwanted astigmatism at the edges of the lens. To counteract this, new wearers must learn to move their head to point their nose directly at the object they wish to focus on, rather than just moving their eyes. This technique ensures that the central, clear part of the lens is used for viewing.
Safety considerations are part of the initial adjustment, especially when navigating vertical spaces. When walking down stairs, it is beneficial to consciously lower the chin to look through the distance portion of the lens, rather than the magnifying near portion. Consistent, full-time wear is the most effective way to help the eyes and brain quickly acclimate.