Lined bifocals are a type of corrective eyewear that addresses presbyopia, the age-related condition that causes difficulty focusing on close objects. These lenses provide two separate focal lengths, allowing the wearer to see clearly at both far and near distances. This design eliminates the need to constantly switch between distance glasses and reading glasses.
The Defining Visual Feature
The most recognizable characteristic of a lined bifocal is the visible line that horizontally separates the lens into two sections. This demarcation is the reason these lenses are often referred to as “traditional” or “lined” bifocals. The larger, upper portion of the lens remains smooth and seamless, dedicated to the distance prescription. The smaller, lower portion contains the specialized segment for near vision, which is clearly outlined by the line. The visible line does not span the entire width of the lens in all styles, but it always marks the boundary of the near-vision segment. This segment is typically positioned lower on the lens, corresponding to the natural downward gaze used for reading or close work. This sharp boundary distinguishes a lined bifocal from other, more modern types of multifocal lenses.
Understanding the Dual Power Zones
The lens is fundamentally divided into two zones, each containing a single, fixed prescription power. The upper zone is calibrated to correct for distance vision, which is used for activities like driving or walking. The distinct segment in the lower zone holds the “add” power, a magnification necessary for clear near vision, such as reading a book or viewing a phone screen. The line itself represents an abrupt shift in optical power, not a gradual one; when the wearer’s line of sight crosses this boundary, the prescription power immediately changes from the distance correction to the near correction. This immediate change is what allows for a wide, undistorted field of view in both the distance and the near zones, as there is no blurred intermediate area.
Common Segment Shapes
Lined bifocals are available in a few distinct segment shapes, which determine the exact appearance of the near-vision area.
D-Segment
The most common design is the D-segment, also known as the Flat-Top or Straight-Top, which looks like a capital letter D lying on its side. These D-shaped segments are frequently sold in widths of 28 millimeters or 35 millimeters, providing a wide area for reading.
Round Segment
Another variation is the Round segment, which features a completely circular near-vision zone at the bottom of the lens. While less common today, this shape was an earlier design that offered a smoother visual edge.
Executive Bifocal
The Executive bifocal, sometimes called the Franklin segment, is the most visually dramatic, as the dividing line runs horizontally across the entire width of the lens. This executive style provides the largest possible field of view for close-up tasks.
Lined vs. Unlined: The Key Differences
Unlined, or progressive, lenses eliminate the distinct segment, instead using a smooth, continuous gradient of power from the top to the bottom of the lens. This seamless design makes unlined lenses look aesthetically identical to single-vision glasses. Functionally, the line in a traditional bifocal causes a phenomenon known as “image jump” when the eye moves across it. This apparent displacement of objects is due to the immediate, sharp change in magnification at the segment line. Progressive lenses avoid this issue by gradually increasing the prescription power, creating a smooth visual experience for distance, intermediate, and near viewing.