Which bifocal type has the least image jump?

Flat-top (D-segment) and executive bifocals have the least image jump because their segment optical center is located at or very near the top of the segment (at the seg line). This makes the distance d in the jump formula (jump = d x add) nearly zero, regardless of the add power.

Where should the bifocal segment be positioned?

The standard position places the top of the segment at the lower eyelid margin when the patient looks straight ahead. This allows natural downgaze to enter the near zone. The position can be adjusted higher for patients who do extensive near work or lower for patients who primarily need distance vision.

What is the difference between ST-28 and ST-35 flat-top segments?

ST-28 and ST-35 refer to the segment width in millimeters. The ST-28 (28mm wide) is the standard width suitable for most patients. The ST-35 (35mm wide) provides a wider near viewing area, recommended for patients who do extensive near work, have larger PDs, or need more reading field width.

Bifocal Lens Types for ABO

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).

For ABO exam purposes, remember: flat-top has MINIMAL image jump (OC at seg line), round segment has MAXIMUM image jump (OC at circle center), and executive has the WIDEST near field. Flat-top is the most commonly dispensed bifocal type.

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.

When transitioning a patient from progressives to bifocals (or vice versa), explain the key differences: bifocals have a visible line but provide wide, distortion-free distance and near zones. Progressives have no line but have peripheral distortion. Some patients genuinely prefer bifocals for their wider near zone and predictable optics.

Setting the seg height too low. If the segment is too far below the pupil, the patient must look down excessively to find the near zone, causing neck strain. Conversely, if it is too high, the seg line intrudes into the distance zone. The lower lid margin is the standard starting point; adjust based on patient needs.

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