What Is Image Jump?
Image jump is the sudden, abrupt apparent displacement of an object when your line of sight crosses the top edge of a bifocal segment. As your gaze moves from the distance portion into the near segment, the image appears to "jump" because you instantly encounter a different prismatic effect at the segment line.
Unlike image swim (a gradual distortion), image jump happens at a single boundary and creates a sharp visual discontinuity.
Why Does It Happen?
Every lens produces prismatic effect at any point away from its optical center (Prentice's Rule). The bifocal segment is essentially a small lens with its own optical center. At the top of the segment, the distance from the segment's optical center determines the prismatic effect.
The amount of image jump depends on two factors:
- Add power of the segment
- Distance from the segment optical center to the top of the segment
Using Prentice's Rule:
Image jump = d × Add
Where d is the distance (in cm) from the segment's optical center to the top of the segment, and Add is the near addition power.
Segment Design and Image Jump
Different bifocal segment designs place their optical center at different locations relative to the segment top. This dramatically affects image jump:
| Segment Type | OC Location | Image Jump |
|---|---|---|
| Round segment (Kryptok) | Center of the circle | Maximum |
| Flat-top (D-segment, ST-28) | At the top of the segment | Minimal |
| Executive (Franklin) | At the seg line (full width) | Minimal |
| Curved-top | Between center and top | Moderate |
Calculating Image Jump
For a round 22 mm segment with a +2.50 D add:
The OC is at the center of the circle, so d = radius = 11 mm = 1.1 cm
Jump = 1.1 × 2.50 = 2.75Δ base up
For a flat-top 28 mm segment with the same +2.50 D add:
The OC is at the seg line, so d = 0 (approximately)
Jump = 0 × 2.50 = 0Δ
The flat-top virtually eliminates image jump regardless of the add power.
Direction of Image Jump
For plus add segments (all standard bifocals), the image jump is base up from the segment. This means objects appear to shift upward as your gaze enters the near zone. The practical effect is that the floor or reading material seems to suddenly move closer as you look down past the seg line.
Clinical Significance
Image jump is particularly bothersome for:
- Patients descending stairs while wearing bifocals
- New bifocal wearers who are not accustomed to the visual shift
- Patients with high add powers (+3.00 D or more)
When a patient complains about a jarring visual shift at the segment line, switching from a round segment to a flat-top design can resolve the issue without changing the prescription.
Key Takeaways
- Image jump is a sudden apparent displacement at the bifocal segment boundary
- It depends on add power and the distance from segment OC to segment top
- Flat-top segments minimize image jump (OC at seg line)
- Round segments produce maximum image jump (OC at circle center)
- The direction is always base up for plus-add segments