Bifocal Types & Segment Styles: Complete Guide for ABO Exam

Why Bifocal Segments Matter for Your ABO Exam

Bifocals are foundational knowledge for any optician. Even though progressives dominate the market today, bifocals remain popular for specific patient needs—wide reading areas, clear segment boundaries, lower cost, and straightforward adaptation. The ABO exam tests bifocal segments heavily, with 10-15 questions covering segment types, dimensions, optical center locations, segment height measurement, and when to recommend each style.

What makes bifocals tricky is the variety. There's flat-top 28, flat-top 35, round 22, round 24, Executive, Ultex, ribbon segments—each with different dimensions, optical properties, and ideal use cases. You need to know which segment provides the widest reading area, which has image jump, where the optical center sits, and how to measure segment height correctly. Miss these details and you'll struggle on exam day.

The exam loves scenario questions: "Patient needs wide reading area for sheet music—which segment?" or "What's the segment height for a flat-top 28 if you want the top line to sit at the lower pupil margin?" These aren't trick questions if you understand segment geometry. But if you've only memorized names without understanding the optical principles, you'll guess wrong.

In this guide, you'll learn every major bifocal segment type, their dimensions and optical characteristics, where optical centers sit, how to measure segment height, the differences between one-piece and fused bifocals, and which segments work best for specific patient needs. By the end, you'll recognize any segment style the ABO throws at you and know exactly when to recommend it.

What are Bifocal Lenses?

Bifocal lenses contain two distinct optical powers in one lens: distance power in the upper portion and near power (reading addition) in the lower segment. The segment has a visible line that separates distance from near. When your eyes look through the upper portion, you see distance clearly. Drop your gaze below the segment line, and you see near clearly.

Clinical Purpose

Bifocals solve presbyopia—the age-related loss of accommodation that makes reading difficult after age 40. Your crystalline lens stiffens, you lose the ability to focus up close, and you need plus power for near tasks. Instead of carrying separate reading glasses, bifocals put distance and near correction in one pair.

The segment line is both a benefit and a drawback. Benefit: clear visual boundary between distance and near zones. Drawback: visible cosmetic line and potential image jump when crossing the segment line. Patients who want invisible transitions choose progressives. Patients who want clarity and simplicity choose bifocals.

One-Piece vs Fused Bifocals

Fused bifocals have a separate piece of high-index glass fused into the front surface of the carrier lens. The segment glass has higher refractive index than the carrier, creating the reading power. Fused bifocals offer excellent optical quality but are heavier and limited to glass materials. They're less common today but still show up on the ABO exam.

One-piece bifocals are molded or ground from a single piece of material—either glass or plastic. The segment is ground into the lens surface. They're lighter than fused bifocals and can be made in any lens material (CR-39, polycarbonate, Trivex, high-index). Most modern bifocals are one-piece designs. The ABO expects you to know the difference and identify which construction method applies to specific segment types.

Major Bifocal Segment Types

Here are the segment types you'll see on the ABO exam, their dimensions, characteristics, and when to use them.

Flat-Top (D-Seg) Bifocals

Flat-top bifocals have a flat horizontal top line with a rounded bottom. They're called "D-seg" because the segment shape resembles the letter D. Flat-tops are the most popular bifocal style today—over 80% of bifocal prescriptions use flat-top segments. Why? They provide a wide reading area, good cosmetics, and easy adaptation.

Flat-Top 28 (FT-28): The segment is 28mm wide. This is the standard bifocal segment. The optical center of the segment sits 5mm below the segment line. FT-28 provides sufficient reading width for most tasks—books, phones, computer keyboards. It's the default choice unless the patient needs something wider or narrower.

Flat-Top 35 (FT-35): The segment is 35mm wide—7mm wider than FT-28. The optical center sits 5mm below the segment line (same as FT-28). FT-35 provides a wider reading zone, ideal for patients who read sheet music, blueprints, spreadsheets, or do extensive desk work. The wider segment also means more peripheral distortion in the near zone compared to FT-28.

Flat-Top 25 (FT-25): Less common, but available for smaller frames or cosmetic preferences. The segment is 25mm wide with the optical center 5mm below the line. Narrower reading zone but less visible from the front.

Round Segment Bifocals

Round segments (also called "button" segments) are circular. They were the original bifocal design and remain common in certain applications. Round segments come in several diameters—22mm, 24mm, and 28mm are most common. The optical center sits at the geometric center of the circle.

Round 22 (R-22): 22mm diameter circle. Small reading area, minimal cosmetic visibility. Often used in safety glasses or when patient wants discreet bifocal. The segment optical center is 11mm below the top of the circle (center of 22mm diameter).

Round 24 (R-24): 24mm diameter. Slightly larger than R-22, still relatively small. Same principle—optical center at circle center, which is 12mm below the top of the segment.

Round segments have more image jump than flat-tops because the segment optical center sits lower relative to the segment top. They also provide narrower reading zones compared to flat-tops of similar width. Today, round segments are less popular except for specific occupational or cosmetic needs.

Executive (Franklin) Bifocal

Executive bifocals (also called Franklin or E-style) have a segment line that extends across the entire lens width. The segment occupies the entire lower half of the lens. This provides the widest possible reading area—edge to edge—making Executives ideal for patients who need maximum near vision width.

The segment line is a straight horizontal line across the lens, typically placed at or just below the lower pupil margin. The segment optical center sits on the segment line (unlike flat-tops where it's 5mm below). This reduces image jump compared to round segments but doesn't eliminate it entirely.

Who needs Executives? Patients who do wide-field near work—accountants with large ledgers, architects with blueprints, musicians reading full-width sheet music. The trade-off is cosmetics: the segment line is highly visible from the front. Most patients prefer the smaller, more discreet flat-top segments unless they specifically need the extra width.

Ultex (Univis) Bifocal

Ultex bifocals are a one-piece design with a subtle segment line and near zone blended into the lower lens. They provide a seamless look with less visible line than traditional segments. The segment shape is similar to flat-top but with softer, more gradual edges. Ultex segments come in various widths (similar to flat-tops).

Ultex segments are popular for patients who want bifocal functionality but dislike the visible line. They bridge the gap between traditional bifocals and progressives—clearer segment boundaries than progressives, subtler line than standard flat-tops. The optical characteristics are similar to flat-tops with segment OC 5mm below the line.

Ribbon Segment (Curved-Top) Bifocal

Ribbon segments have a curved top line that follows the natural curve of the lower eyelid. This creates a more natural look and reduces the visible line when viewed from the front. Ribbon segments are less common today but still appear on ABO exams as a historical reference.

How to Measure Segment Height

Segment height is the vertical distance from the bottom of the lens (where it sits in the frame) up to the top of the segment line. This measurement determines where the reading zone starts. Get it wrong and the patient will struggle—segment too high and they see reading power when looking straight ahead; segment too low and they have to drop their gaze uncomfortably far to read.

Standard Measurement Procedure

Have the patient wear the frame. Adjust it so it sits naturally—no sliding, no tilting. Patient stands or sits looking straight ahead at eye level (primary gaze). Measure the distance from the lowest point of the lens opening (inside the frame groove) up to the lower pupil margin. That's your starting point for most bifocals.

For flat-top bifocals, the segment line typically sits at or 1-2mm below the lower pupil margin. Why? When you read, your eyes naturally drop about 10-12mm below primary gaze. If the segment line is at the lower pupil margin, dropping your gaze brings your eyes into the segment zone comfortably. Plus, the segment line isn't visible when looking straight ahead—it's hidden by the lower lid.

For Executive bifocals, place the segment line at or just below the lower pupil margin so the patient doesn't see reading power in primary gaze but has easy access when looking down. For round segments, consider that the optical center sits at the circle's center—measure accordingly to ensure the OC aligns with the patient's near PD when reading.

Tools for Measurement

Use a millimeter ruler, segment height gauge, or pupilometer with vertical measurement capability. Mark the lower pupil margin on the demo lens with a grease pencil. Measure from the bottom of the lens opening to your mark. That's your segment height. Most bifocals have segment heights between 14mm and 20mm depending on frame size and patient preference.

Common Measurement Mistakes

Measuring with the frame tilted, patient looking down, or measuring to the pupil center instead of lower margin. Always measure with the patient in primary gaze and the frame adjusted properly. Also, don't forget to account for frame position—if the frame will sit lower on the patient's face than it does during measurement, adjust your segment height accordingly.

Optical Center Placement in Bifocal Segments

Every bifocal segment has an optical center (OC)—the point of zero prismatic effect within the segment. The location of this OC relative to the segment line affects optical performance and image jump. The ABO tests this concept repeatedly.

Flat-Top Segments

For FT-28 and FT-35, the segment OC sits 5mm below the segment line. Why 5mm? This is the optimal distance for reading comfort. When you drop your gaze to read, your eyes naturally converge and look through a point about 5mm below where the segment starts. Placing the OC there aligns with your natural reading posture and minimizes unnecessary prism.

Round Segments

For round segments, the OC sits at the geometric center of the circle. R-22 has its OC 11mm below the top of the segment (half of 22mm). R-24 has its OC 12mm below the top (half of 24mm). This lower OC position (compared to flat-tops) means more image jump when crossing the segment line—one reason round segments feel less comfortable for some patients.

Executive Segments

Executive segments have the OC right on the segment line. This reduces image jump compared to round segments because the prismatic discontinuity is smaller. However, Executives still have some image jump—less than round segments but more than flat-tops—because the segment line itself creates a boundary between two different powers.

Why OC Location Matters

The farther the segment OC is from the segment line, the more prism difference exists at the line, and the more image jump you get. Flat-tops minimize this with the 5mm offset. Round segments have larger offsets, creating more jump. Understanding this relationship helps you explain to patients why flat-tops adapt more easily than round segments.

For a deeper dive into image jump and its causes, see our complete guide on Image Jump in Bifocals.

How the ABO Exam Tests Bifocal Segments

The ABO dedicates 10-15 questions to bifocal segments, covering identification, measurement, optical properties, and patient selection. Here's what to expect and how to prepare.

Question Types

Segment Identification: "Which bifocal segment provides the widest reading area?" Answer: Executive. "What is the segment width of FT-28?" Answer: 28mm. These are straightforward if you've memorized the dimensions.

Optical Center Questions: "Where is the optical center of an FT-28 segment located?" Answer: 5mm below the segment line. "Where is the OC of a round 22 segment?" Answer: At the geometric center, 11mm below the top.

Segment Height Scenarios: "Patient complains bifocal segment line is visible when looking straight ahead. What's the problem?" Answer: Segment height too high—should be at or below lower pupil margin. These test your understanding of proper fitting technique.

Patient Selection: "Which bifocal is best for a musician who reads sheet music?" Answer: Executive, because it provides the widest reading area. "Which segment is most cosmetically appealing?" Answer: Flat-top or Ultex, because they have less visible lines than round or Executive segments.

Study Tips

Create a comparison chart with all segment types: name, width/diameter, OC location, image jump characteristics, ideal use case. Memorize the FT-28 and FT-35 dimensions cold—these are the most tested. Know that flat-tops have OC 5mm below the line, round segments have OC at center, and Executive has OC on the line.

Practice measuring segment height on every bifocal you encounter. The more you practice, the faster you'll recognize correct and incorrect measurements on the exam. When you see a segment height question, visualize the frame on a patient's face and think about where the segment line would sit.

Link bifocal segments to other concepts: image jump, prism, near PD, pantoscopic tilt. The exam likes integrated questions that test multiple concepts at once. If you understand how segment design affects image jump, you can answer both segment and prism questions confidently.

ABO Practice Questions

Test your bifocal segment knowledge with these ABO-style questions. Try to answer before revealing the solutions.

Related ABO Topics

Bifocal segments connect to several other ABO concepts. Review these topics to strengthen your understanding: