ABO NOCE Basic Opticianry Bifocal and Trifocal Lens Design

Question 1

What is 'image jump' in a bifocal lens?

Accepted Answer

The prismatic displacement of an image that occurs as the line of sight crosses the top edge of the bifocal segment

Answer

The increase in add power as gaze moves downward into the segment

Answer

The chromatic aberration produced at the segment boundary

Answer

The change in apparent magnification between distance and near zones

Question 2

Which bifocal style produces zero image jump?

Accepted Answer

Executive (Franklin split) bifocal

Answer

Flat-top (D-segment) bifocal

Answer

Round-segment bifocal

Answer

Ultex (curved-top) bifocal

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?

Accepted Answer

1.25 prism diopters

Answer

0.50 prism diopters

Answer

2.50 prism diopters

Answer

0 prism diopters

Question 4

The intermediate segment of a trifocal lens is typically prescribed at what power relative to the full near add?

Accepted Answer

One-half of the full add

Answer

One-third of the full add

Answer

Two-thirds of the full add

Answer

Equal to the full add

Question 5

Seg height for a bifocal is typically measured from which reference point?

Accepted Answer

From the lowest point of the lens (frame bottom) to the top of the segment dividing line

Answer

From the top of the frame to the segment dividing line

Answer

From the center of the pupil to the top of the frame

Answer

From the bottom of the pupil to the segment dividing line

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?

Accepted Answer

The executive bifocal's segment OC is at the dividing line, so Prentice's Rule produces zero prism at segment entry

Answer

The executive bifocal has a lower add power, reducing the prismatic effect

Answer

The executive bifocal has no optical center, so no prism is produced

Answer

The wider segment distributes prism over a larger area, making it imperceptible