Image Jump in Bifocals: Complete Guide for ABO Exam

Why Image Jump Matters for Your ABO Exam

Image jump is that annoying visual "hop" patients experience when their eyes cross the bifocal segment line. Objects appear to jump up or down as the line of sight moves from distance to near power. It's a common complaint with bifocals and a heavily tested concept on the ABO exam. You'll see 5-8 questions covering what causes image jump, which segment types have more or less jump, how to calculate the amount of jump, and how to minimize it.

Image jump happens because of prism. When you cross the bifocal segment line, your line of sight suddenly passes through a different optical center with different prismatic effects. The abrupt change in prism creates a perceived displacement of the image—that's the jump. The farther the segment optical center is from the segment line, the more prism difference exists at the boundary, and the more jump you get.

Here's what makes image jump tricky on the exam: it's not just memorization—you need to understand the optical principles. The ABO will give you scenarios: "Patient complains of excessive image jump with round 22 bifocals. Why?" or "Which bifocal segment minimizes image jump?" If you understand how segment design, optical center placement, and prism interact, these questions become straightforward. If you've only memorized segment names without understanding the optics, you'll struggle.

In this guide, you'll learn what image jump is, why it happens (prismatic discontinuity at the segment line), how to calculate the amount of jump using Prentice's rule, which segment types have more or less jump, why Executive bifocals minimize jump, why progressive lenses eliminate it entirely, and practical strategies to help patients adapt. By the end, you'll confidently answer every image jump question the ABO throws at you.

What is Image Jump?

Image jump is the apparent displacement of an object when the line of sight crosses the bifocal segment line. As your eyes move from the distance portion of the lens to the near segment, objects seem to suddenly jump upward or downward. This creates a momentary visual disruption that can be disorienting, especially for new bifocal wearers.

What Patients Experience

When a patient looks straight ahead through the distance portion, everything appears stable. As they drop their gaze to read—crossing the segment line—the page or phone appears to jump upward slightly. It's not the object actually moving; it's a change in the prismatic effect of the lens that causes the brain to perceive displacement. Most patients adapt to this within a few days, but some find it bothersome enough to request progressives instead.

The Optical Cause: Prismatic Discontinuity

Image jump is caused by a difference in prismatic effect between the distance lens and the segment at the segment line. When your line of sight passes through any point on a lens away from the optical center, prism is induced (this is Prentice's rule). The amount of prism depends on how far you are from the optical center and the lens power.

In bifocals, the distance optical center is typically positioned at the distance PD. The segment optical center is positioned lower—at the near PD and below the segment line. When your eyes cross the segment line, they suddenly switch from looking through the distance lens (with its prismatic effect at that point) to looking through the segment (with its prismatic effect at that point). These two prismatic effects are different, creating a discontinuity. Your brain perceives this discontinuity as the image jumping.

Direction of Image Jump

The direction of image jump depends on the lens power and segment design. For plus lenses (hyperopic corrections), the image typically jumps upward when crossing into the segment. For minus lenses (myopic corrections), the image typically jumps downward. The ABO may ask you to predict the direction of jump given a prescription and segment type.

How to Calculate Image Jump

Image jump can be quantified using Prentice's rule, which relates prism to lens power and distance from the optical center. Understanding this calculation helps you predict which segment types will cause more or less jump.

Prentice's Rule Review

Prentice's rule states: Prism (Δ) = Power (D) × Decentration (cm). Prism is measured in prism diopters (Δ), power is the lens power in diopters (D), and decentration is the distance from the optical center in centimeters.

For image jump calculations, you need to determine the prismatic effect at the segment line for both the distance lens and the segment, then find the difference. The difference is the image jump.

Image Jump Formula

Image Jump = (Segment Add Power) × (Distance from segment line to segment OC in cm)

This simplified formula works because image jump is primarily caused by the segment add power and the vertical distance between the segment line (where your eyes cross) and the segment optical center. The farther the segment OC is from the segment line, the more prism difference exists at the line, and the more jump you get.

Example Calculation: Flat-Top 28

Let's calculate image jump for a +2.00 D add in an FT-28 bifocal. The segment optical center in an FT-28 sits 5mm (0.5 cm) below the segment line.

Image Jump = Add Power × Distance = 2.00 D × 0.5 cm = 1.0Δ

So a +2.00 add in an FT-28 creates 1 prism diopter of image jump. For a +2.50 add, the jump would be 1.25Δ. The higher the add, the more jump.

Example Calculation: Round 22

For a round 22 segment, the optical center sits at the geometric center of the 22mm circle—11mm (1.1 cm) below the top of the segment. Using a +2.00 add:

Image Jump = 2.00 D × 1.1 cm = 2.2Δ

Round segments have more than twice the image jump of flat-tops because the OC is farther from the segment line. This is why round segments feel "jumpier" and why flat-tops are more popular for patient comfort.

Which Bifocal Segments Have More Image Jump?

Not all bifocal segments create equal amounts of image jump. The key factor is the distance from the segment line to the segment optical center. The farther apart these are, the more jump you get.

Flat-Top Bifocals (Minimal Jump)

Flat-top segments (FT-28, FT-35, FT-25) have the segment OC just 5mm (0.5 cm) below the segment line. This close proximity minimizes the prismatic discontinuity at the line, resulting in relatively small image jump. For typical add powers (+1.50 to +2.50 D), flat-tops produce 0.75Δ to 1.25Δ of jump—noticeable but tolerable for most patients.

This is the primary reason flat-tops dominate the bifocal market. They balance reading area width with minimal image jump, making adaptation easier. When patients ask, "Which bifocal has the least jump without going to progressives?" the answer is flat-top.

Round Segments (More Jump)

Round segments have the optical center at the geometric center of the circle. For R-22, that's 11mm below the top. For R-24, it's 12mm below. This places the OC much farther from the segment line compared to flat-tops, resulting in significantly more image jump—often 2-2.5Δ for typical add powers.

Patients notice this jump more with round segments. It's one reason round segments have fallen out of favor except for specific applications like safety glasses (where small segment size is desirable) or when cosmetic discretion is the priority.

Executive Bifocals (Least Jump Among Traditional Bifocals)

Executive bifocals have the segment optical center right on the segment line. This means the distance from line to OC is zero (or very close to zero), resulting in minimal image jump—theoretically approaching zero. In practice, there's still a small prismatic discontinuity because the segment line separates two different powers, but it's far less noticeable than flat-tops or round segments.

If a patient complains about image jump with flat-tops and needs bifocals (not progressives), Executive bifocals are the next step. The wide segment line is more visible, but the jump is minimal. This is a key ABO exam concept: Executive bifocals minimize image jump.

Progressive Lenses (Zero Jump)

Progressive addition lenses eliminate image jump entirely. Why? No segment line means no prismatic discontinuity. Power changes gradually from distance to near, so there's no abrupt boundary to cross. Your eyes experience a smooth transition with no jump.

This is one of the main selling points of progressives: "No lines, no jump." Patients who are sensitive to image jump and can afford progressives almost always prefer them over bifocals. The ABO expects you to know that progressives eliminate image jump, while all traditional bifocals (even Executives) have at least some jump.

How to Minimize Image Jump

If a patient struggles with image jump and you need to stick with bifocals (not progressives), here are strategies to reduce the problem.

1. Choose Segment Type with OC Closer to Segment Line

Switch from round segments to flat-tops, or from flat-tops to Executive bifocals. Executive bifocals have the OC on the line, minimizing jump. Flat-tops have OC 5mm below, which is better than round segments (OC 11-12mm below). This is the most effective strategy short of switching to progressives.

2. Reduce Add Power (If Clinically Appropriate)

Image jump increases with add power (jump = add × distance). If the prescribing doctor can reduce the add from +2.50 to +2.00, the jump decreases proportionally. However, you can't arbitrarily change the prescription—this requires doctor approval. Only suggest this if the patient is over-added (reading power stronger than necessary).

3. Proper Segment Height Placement

Ensure the segment line sits at or just below the lower pupil margin so it's hidden by the lower eyelid in primary gaze. If patients don't see the segment line when looking straight ahead, they're less aware of crossing it when they drop their gaze to read. This doesn't reduce the optical jump, but it reduces the psychological awareness of it.

4. Patient Education and Adaptation

Warn patients upfront that they'll notice image jump for the first few days but most people adapt within a week. Teach them to move their head (not just their eyes) when switching between distance and near, which makes the transition smoother. Set expectations and most patients adapt fine. Surprise them with image jump and they'll panic.

5. Recommend Progressive Lenses

If image jump is truly intolerable and the above strategies don't help, progressives are the definitive solution. They eliminate jump entirely. The trade-off is cost, adaptation period, and peripheral distortion in progressives. But for patients sensitive to image jump, progressives are worth it.

For more on progressive lens benefits and fitting, see our complete guide on Progressive Addition Lenses.

How the ABO Exam Tests Image Jump

The ABO includes 5-8 questions on image jump, covering causes, calculations, segment comparisons, and minimization strategies. Here's what to expect and how to prepare.

Question Types

Cause Questions: "What causes image jump in bifocal lenses?" Answer: Prismatic discontinuity at the segment line caused by the difference in optical center locations between distance and segment. "Why do progressive lenses not have image jump?" Answer: No segment line, no discontinuity.

Segment Comparison: "Which bifocal segment has the most image jump?" Answer: Round segments (OC farthest from line). "Which bifocal minimizes image jump?" Answer: Executive (OC on the line). These test your understanding of segment optical center placement.

Calculation Questions: "Calculate image jump for a +2.50 add in an FT-28 bifocal." Answer: 2.50 × 0.5 = 1.25Δ. Know the OC distances: FT = 0.5 cm, R-22 = 1.1 cm, Executive = 0 cm. Apply Prentice's rule.

Patient Scenarios: "Patient complains of excessive image jump with current bifocals. What segment change would reduce this?" Answer: Switch from round to flat-top, or from flat-top to Executive. "Patient asks for bifocals with no image jump." Answer: No bifocal eliminates jump entirely; recommend progressives.

Study Tips

Memorize segment OC locations: FT = 5mm below line, Round = center of circle (11-12mm below top), Executive = on the line. These numbers are the foundation for every image jump calculation and comparison question.

Practice Prentice's rule calculations. The ABO loves giving you an add power and segment type, then asking you to calculate jump. Do 10-15 practice problems with different add powers and segment types until the math is automatic.

Understand the relationship: more distance from line to OC = more jump. Higher add power = more jump. This conceptual understanding lets you answer qualitative questions (which segment has more jump?) without calculating every time.

ABO Practice Questions

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

Related ABO Topics

Image jump connects to several other ABO concepts. Review these topics to strengthen your understanding: