Bifocal lenses have two distinct optical powers within a single lens, correcting distance vision and the near vision difficulties associated with presbyopia. This age-related condition causes the eye’s natural lens to lose flexibility, making close-up tasks like reading a challenge. As vision changes, many people seek to update their current single-vision glasses by adding reading power. The direct modification of an existing lens to incorporate a bifocal segment is generally not possible, due to the technical process of lens manufacturing.
Why Existing Lenses Cannot Be Modified
The definitive answer to modifying existing lenses is no, because the multifocal design is an integral part of the lens structure created during fabrication. A standard single-vision lens has a uniform curvature to correct a single refractive error. The bifocal segment, known as the “add power,” must be incorporated into the lens blank before the final grinding, polishing, and coating processes.
For plastic lenses, which constitute the majority of modern eyewear, the bifocal reading power is either molded into the lens blank or created through surfacing. This involves grinding a specific, stronger curve onto the lens surface using computer-controlled tools, which is then polished smooth. The power difference is established deep within the lens material itself, not as a surface application.
For traditional glass bifocals, the stronger-powered reading segment is a separate piece of glass fused onto the main lens blank using heat before shaping. Once the lens is completed, treated with coatings, and cut to fit the frame, its optical properties are permanently fixed. Attempting to grind a new curve or fuse a new segment onto a finished lens would destroy the existing optical integrity and surface coatings.
The Stick-On Alternative
While permanently altering a finished lens is impossible, a temporary solution exists: stick-on bifocal segments. These are thin, flexible half-moon-shaped wafers, typically made of soft plastic or silicone, applied to the inside surface of existing lenses. The material adheres using water-based surface tension or static cling, eliminating the need for permanent adhesives.
These segments function as a separate positive-power lens, providing magnification for reading or close-up work. They are often used in non-prescription eyewear like sunglasses or safety glasses, or as a temporary, budget-conscious solution. Users can trim the segments to fit their frame shape and position them lower on the lens to mimic a traditional bifocal segment.
The main drawbacks of this alternative are related to optical quality and maintenance. Because they are separate layers, stick-on segments can introduce slight visual distortion or unwanted prismatic effects, and the segment line is always visible. They also require careful handling during cleaning to ensure they do not shift or peel away from the lens surface.
Replacing Lenses: Bifocals Versus Progressives
When vision changes, the only way to gain bifocal functionality is to replace the lenses entirely with new multifocal lenses; however, existing frames can often be reused. When ordering new lenses, you choose between traditional bifocals and modern Progressive Addition Lenses (PALs). Traditional bifocals feature a clear, visible line separating the distance-vision area from the smaller, lower reading segment.
The primary advantage of traditional bifocals is the wide, clear field of vision in both distance and near zones, but the distinct line causes an “image jump” when the eye shifts focus. PALs, conversely, offer a seamless, line-free transition of power that gradually changes from distance correction at the top to near correction at the bottom. This continuous change more closely mimics natural vision and eliminates the sudden shift in image location.
Progressive lenses include an intermediate viewing zone, which is beneficial for tasks like computer use or viewing a car dashboard. While PALs offer a more natural viewing experience, they have areas of peripheral distortion on the sides of the lens corridor and typically require a brief adjustment period. Both options allow you to retain your frame, provided it is in good condition and has sufficient vertical depth to accommodate the viewing zones.