What Are Bifocal Lenses?
Bifocal lenses provide two distinct optical zones: a distance portion (upper area) and a near addition (lower segment). The near segment has additional plus power (the "add") that compensates for the presbyopic patient's loss of accommodation. Benjamin Franklin is credited with inventing the first bifocal in 1784.
Common Bifocal Segment Types
| Type | Segment Shape | OC Location | Image Jump | Field of View |
|---|---|---|---|---|
| Flat-top (D-segment) | Semicircular with flat top | At the seg line | Minimal | Moderate (28mm or 35mm wide) |
| Round segment | Circular (22mm or 25mm) | Center of circle | Maximum | Small to moderate |
| Executive (Franklin) | Full-width line across lens | At the seg line | Minimal | Maximum (full lens width) |
| Curve-top | Curved upper edge | Between top and center | Moderate | Moderate |
Flat-Top (D-Segment)
The flat-top (also called D-segment, straight-top, or ST) is the most commonly dispensed bifocal. Available in 28mm (ST-28) and 35mm (ST-35) widths. Its key advantage is minimal image jump because the segment's optical center is located at the very top of the segment.
Round Segment
The round segment (Kryptok or Ultex) has a circular near zone. The optical center is at the center of the circle, far from the top edge, causing maximum image jump. Less commonly prescribed today but still tested on the ABO exam.
Executive (Franklin)
The executive bifocal (also called E-line or Franklin style) has a segment that extends the full width of the lens. It provides the widest near reading area but has a very visible line and adds weight to the lens (the entire lower half has the add power).
Fitting Bifocal Lenses
Segment Height
The segment height (seg height) is the vertical distance from the lowest point of the lens (bottom of the frame) to the top of the bifocal segment. Standard placement:
- Standard: At or just below the lower lid margin
- Higher placement: For patients who do significant near work or have limited downgaze ability
- Lower placement: For patients who primarily need distance vision and only occasionally read
Segment Width
- 28mm (ST-28): Standard width; adequate for most patients
- 35mm (ST-35): Wider near zone; good for patients doing extensive near work or those with larger PDs
Optical Characteristics
Image Jump
Image jump (sudden displacement at the seg line) depends on the distance from the seg OC to the top of the segment. Flat-top and executive minimize this because their OCs are at or near the seg line.
Image Displacement
Image displacement is different from image jump. It is the total prismatic effect at the reading position (below the distance OC) from BOTH the distance lens and the near segment combined. It depends on the distance prescription and the add power.
Key Takeaways
- Bifocals have two zones: distance (upper) and near (lower segment)
- Flat-top (D-segment) is the most common type with minimal image jump
- Round segment has maximum image jump; executive has the widest near field
- Segment height is measured from the bottom of the lens to the top of the segment
- Standard seg height is at the lower lid margin, adjusted for patient needs