Bifocal Dispensing: Segment Height, Width, and Image Jump

Segment Height Placement

Segment height is the vertical distance from the lowest point of the lens in the frame to the top of the bifocal segment line. Correct placement ensures the patient can access the near zone comfortably without excessive head movement.

The standard guideline for most flat-top bifocals is to place the segment top at the lower eyelid margin. This positions the near zone just below the patient's natural line of sight, keeping the distance zone unobstructed during straight-ahead viewing while making the near zone easily accessible by lowering the eyes.

Measuring Segment Height

1. Fit the frame to the patient with proper adjustments (tilt, bridge, temples)
2. Have the patient look straight ahead at a distant target
3. Mark the position of the lower eyelid on the demo lens or use a ruler
4. Measure from the lowest point of the frame's lens groove to this mark
5. This measurement is the segment height you order from the lab

When to Deviate from Standard Placement

Not every patient benefits from standard lower-lid segment placement:

• Occupational use: A plumber or electrician working overhead may need the segment lower to prevent the near zone from interfering with overhead distance viewing
• Desk workers: Patients who spend most of their time reading may prefer a slightly higher segment for easier access to the near zone
• Previous bifocal wearers: Match the segment height of their current glasses if they are happy with it, even if it deviates from the standard guideline

Segment Width Selection

The segment width determines how much of the lower lens area is dedicated to near vision. Common widths include:

The FT-28 is the most frequently dispensed bifocal segment. It provides a generous near zone for typical reading tasks while keeping the segment visually unobtrusive. The FT-35 offers a wider reading area, which benefits patients who read large-format materials or work at desks with spread-out documents.

The executive (Franklin) bifocal uses a full-width segment that extends across the entire lens. This provides the widest possible near field but makes the segment line very visible. Executive bifocals are common in occupational settings where maximum near coverage is needed.

Image Jump

Image jump is the sudden apparent displacement of an object as the patient's line of sight crosses from the distance portion into the near segment. This occurs because the segment introduces a prismatic effect at the segment line, causing the image to appear to "jump" to a new position.

The amount of image jump depends on where the optical center of the near segment is located relative to the segment top:

• Flat-top segments: The optical center of the near portion is located at the bottom of the segment, far from the segment top. This creates noticeable image jump as the line of sight crosses the segment line.
• Round segments: The optical center is at the center of the circular segment, which is some distance below the segment top. Image jump is moderate.
• Executive segments: The optical center of the near portion falls at the segment line itself. This means zero image jump, which is a significant advantage of this design.

Patient Adaptation

Most patients adapt to bifocals quickly, especially those who have never worn multifocals before (they have no progressive habits to unlearn). Key adaptation advice:

• Head movement: Lower your chin to read rather than just dropping your eyes. This keeps the near segment in front of your eyes.
• Stairs: The near segment sits in the lower portion of the lens, which is where you naturally look when walking downstairs. Tell patients to lower their chin slightly when descending stairs to look through the distance portion.
• Segment line awareness: Patients will notice the visible line initially. This awareness typically fades within a few days as the brain learns to ignore it.
• Driving: The near segment does not interfere with distance driving since it sits below the normal line of sight. Checking mirrors and the dashboard is unaffected.

Bifocal vs. Progressive: When to Recommend Each

Bifocals remain a valid choice for many patients:

• Patients who want a wide, clear near zone without peripheral distortion
• Patients who have tried progressives and could not adapt
• Budget-conscious patients (bifocals are typically less expensive)
• Patients with specific occupational needs where a wide near zone is critical
• Patients who do not need intermediate vision correction

Progressives are preferred when patients want no visible line, need intermediate vision, or prefer a cosmetically modern appearance.

Key Takeaways

• Standard segment height placement is at the lower eyelid margin
• FT-28 is the most common segment width; wider segments provide more near coverage
• Image jump occurs at the segment line and is greatest with flat-top designs, zero with executive bifocals
• Always fit the frame before measuring segment height
• Patient adaptation to bifocals is typically faster than progressive adaptation
• Bifocals remain appropriate for patients needing wide near zones or unable to adapt to progressives