ABO NOCE Basic Opticianry Bifocal and Trifocal Lens Design — Questions and Answers
Question 1: What is 'image jump' in a bifocal lens?
- The increase in add power as gaze moves downward into the segment
- The prismatic displacement of an image that occurs as the line of sight crosses the top edge of the bifocal segment (Correct answer)
- The chromatic aberration produced at the segment boundary
- The change in apparent magnification between distance and near zones
Correct answer: The prismatic displacement of an image that occurs as the line of sight crosses the top edge of the bifocal segment
Image jump is the sudden prismatic displacement of an object seen the instant the line of sight crosses the top of the bifocal segment. Its magnitude equals the add power multiplied by the distance (in cm) from the segment top to the segment's optical center, per Prentice's Rule.
Question 2: Which bifocal style produces zero image jump?
- Flat-top (D-segment) bifocal
- Round-segment bifocal
- Executive (Franklin split) bifocal (Correct answer)
- Ultex (curved-top) bifocal
Correct answer: Executive (Franklin split) bifocal
In an executive (E-style or Franklin split) bifocal, the segment's optical center is located precisely at the dividing line — the top of the segment. Because the distance from the segment top to its OC is zero, Prentice's Rule yields zero prism at the boundary, eliminating image jump entirely.
Question 3: A flat-top 28 bifocal has an add power of +2.50 D. The segment OC is 5 mm below the segment top. What is the amount of image jump?
- 0.50 prism diopters
- 1.25 prism diopters (Correct answer)
- 2.50 prism diopters
- 0 prism diopters
Correct answer: 1.25 prism diopters
Using Prentice's Rule: P = F × d = 2.50 × 0.5 cm = 1.25 prism diopters. The distance d is 5 mm (0.5 cm) from the segment top to the segment OC, and F is the add power. This base-down prism causes the image to jump upward as gaze enters the segment.
Question 4: The intermediate segment of a trifocal lens is typically prescribed at what power relative to the full near add?
- One-third of the full add
- One-half of the full add (Correct answer)
- Two-thirds of the full add
- Equal to the full add
Correct answer: One-half of the full add
The intermediate addition in a trifocal is half the full near add power. For example, a +2.50 near add produces a +1.25 intermediate add, providing clear focus at arm's length (approximately 60–80 cm). This half-add convention is industry standard for trifocal designs.
Question 5: Seg height for a bifocal is typically measured from which reference point?
- From the top of the frame to the segment dividing line
- From the center of the pupil to the top of the frame
- From the lowest point of the lens (frame bottom) to the top of the segment dividing line (Correct answer)
- From the bottom of the pupil to the segment dividing line
Correct answer: From the lowest point of the lens (frame bottom) to the top of the segment dividing line
Bifocal segment height (seg height) is measured from the lowest edge of the lens or frame opening to the top of the segment dividing line. This measurement tells the lab exactly where to place the segment boundary within the lens blank, ensuring the reading zone is at the correct position for the patient.
Question 6: A patient with a +2.00 add complains of a 'jump' when looking from distance to their bifocal. The optician recommends switching to an executive (E-style) bifocal. Why does this eliminate the jump?
- The executive bifocal has a lower add power, reducing the prismatic effect
- The executive bifocal's segment OC is at the dividing line, so Prentice's Rule produces zero prism at segment entry (Correct answer)
- The executive bifocal has no optical center, so no prism is produced
- The wider segment distributes prism over a larger area, making it imperceptible
Correct answer: The executive bifocal's segment OC is at the dividing line, so Prentice's Rule produces zero prism at segment entry
In the executive bifocal, the dividing line is the optical center of the reading segment. At the OC, the distance d in Prentice's Rule (P = F × d) equals zero, so P = 0 — no prism is induced when the eye first enters the segment, eliminating image jump.
ABO NOCE Basic Opticianry Bifocal and Trifocal Lens Design
What is 'image jump' in a bifocal lens?