7.2 Multifocal & Trifocal Lenses

Design and Optics of Multifocal and Trifocal Lenses

As the human eye ages, it experiences presbyopia—a gradual, natural loss of accommodation (the ability of the ciliary muscle and crystalline lens to increase optical power to focus on near objects). To address presbyopia and restore functional vision at multiple distances, opticians prescribe multifocal and trifocal lenses. Understanding the physical layout, optical dynamics, and fitting protocols for these lenses is vital for optical dispensing and is a primary topic on the NOCE exam.

Bifocal Segment Designs

Bifocal lenses feature two distinct focal zones: a large carrier portion for distance vision and a localized segment (or 'seg') for near vision. The size, shape, and placement of the near segment affect the wearer's visual field, cosmetics, and adaptation. The primary bifocal designs include:

Flat-Top (D-Segment) Lenses

The flat-top (also called the D-segment or straight-top) is the most popular bifocal design. The segment is D-shaped, rotated 90 degrees, presenting a flat horizontal top boundary. The most common widths are FT-28 (28 mm) and FT-35 (35 mm). A key optical characteristic of the flat-top design is that the optical center of the near segment is located exactly 5 mm below the straight top line. Because the optical center is close to the dividing line, the flat-top design minimizes the sudden change in prismatic power as the wearer's gaze transitions from the distance carrier to the near segment.

Round Segment Lenses

In a round segment bifocal, the near zone is a complete circle fused or molded into the carrier lens, typically in diameters of 22 mm or 24 mm. Round segments offer a softer cosmetic appearance because there are no sharp horizontal lines. However, the optical center of a round segment is located at its geometric center (e.g., 11 mm below the top border for a 22 mm segment). This large distance between the top border and the optical center creates a large base-down prismatic effect at the boundary, resulting in a pronounced optical phenomenon known as image jump. Round segments are rarely prescribed for general wear but are used for children with accommodative esotropia (to force near compliance) or for golfers (placed in the upper or outer corner of the lens).

Executive (Franklin) Segment Lenses

The Executive segment (or Franklin style) features a horizontal dividing line extending across the entire width of the lens. The upper half is dedicated to distance vision and the lower half to near vision. The optical center of the near segment lies directly on the dividing line (0 mm distance). While it provides the widest possible reading field (ideal for draftspeople or desk-bound professionals), the Executive design is thick and heavy because the entire lower portion is ground with the near power. The prominent ledge is also highly visible and prone to chipping and collecting debris.

Ribbon Segment Lenses

A ribbon segment is a rectangular strip containing the near prescription, leaving a portion of the distance carrier lens visible both above and below the segment. The standard configurations are the B-segment (9 mm high) and the R-segment (14 mm high). Ribbon segments are specialized occupational lenses designed for individuals who need to see at near while performing tasks below their normal line of sight, such as bricklayers, pilots, or heavy machinery operators who must look down at the floor or ground for distance navigation.

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Trifocal Lens Configurations

For advanced presbyopes who have lost all accommodation, standard bifocals leave a visual gap at intermediate distances (18 to 30 inches), which are critical for computer screens, car dashboards, and sheet music. Trifocal lenses address this by inserting a third, intermediate optical zone directly above the near segment.

Standard Sizes: 7x28 and 8x35

Trifocals are named using their vertical intermediate height and horizontal segment width. The standard configurations are:

• 7x28: A 7 mm high intermediate zone and a 28 mm wide near segment.
• 8x35: An 8 mm high intermediate zone and a 35 mm wide near segment.

Intermediate Power Calculations

The power of the intermediate zone in a standard trifocal is calculated as a percentage of the near addition (Add) power:

• 50% of Add: The industry standard for general-purpose trifocals. If a patient's prescription has a distance sphere of +1.00 D and a near Add of +3.00 D, the intermediate zone power will be +1.50 D (50% of +3.00 D), giving a total intermediate power of +2.50 D (+1.00 D distance + +1.50 D intermediate segment).
• 61% of Add: A specialized percentage used in occupational trifocals designed for patients who work at closer intermediate distances, such as musicians reading close music stands or computer users with specific workstation depths.

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Optical Dynamics: Image Jump vs. Object Displacement

An optician must understand the difference between these two vertical optical effects in multifocal lenses:

Image Jump

Image jump is the sudden vertical displacement of the visual field that occurs the instant the wearer's line of sight crosses the upper boundary of the near segment. This occurs because the optical center of the segment does not lie on the dividing line. As the eye crosses the boundary, it abruptly experiences the base-down prismatic effect of the segment. The magnitude of image jump is independent of the distance lens power and depends solely on the reading Add power (Add) and the distance (d in centimeters) from the segment's top border to the segment's optical center:

Image Jump (in prism diopters) = d * Add

• For an FT-28 bifocal with a +2.00 D Add, the distance d is 0.5 cm (5 mm). The image jump is 0.5 * 2.00 = 1.0 prism diopter base-down.
• For a 22 mm Round Seg with a +2.00 D Add, the distance d is 1.1 cm (11 mm). The image jump is 1.1 * 2.00 = 2.2 prism diopters base-down—more than double the jump of the flat-top.
• For an Executive bifocal, d is 0 cm (0 mm). The image jump is 0 * 2.00 = 0 (no image jump).

Object Displacement (Total Displacement)

Unlike image jump, object displacement (or total displacement) is the total prismatic effect experienced by the wearer at the reading point (typically 8 to 10 mm below the segment line). It is the algebraic sum of the prismatic effect induced by the distance carrier lens (using Prentice's Rule) and the prismatic effect induced by the near segment. In high minus distance prescriptions, the base-down prism of the segment can be neutralized by the base-up prism of the distance lens, reducing total displacement. In high plus distance prescriptions, both the distance lens and the segment induce base-up/base-down components that can compound, increasing total displacement.

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Fitting and Measurement Guidelines

To ensure comfortable vision, the optician must take precise segment height measurements. Before taking any measurements, the frame must be fully adjusted to the patient's face, ensuring proper vertex distance, pantoscopic tilt, and temple fit.

• Bifocal Segment Height: Measured from the lowest point of the inside eyewire groove (the B measurement box bottom) to the lower limbus (the boundary between the colored iris and the white sclera) or the lower eyelid margin. The patient should look straight ahead at the optician's eyes at the same eye level.
• Trifocal Segment Height: Measured from the lowest point of the inside eyewire groove to the lower pupillary margin (the bottom edge of the pupil). This ensures the intermediate zone sits directly in front of the lower pupil, allowing the patient to access it with a slight downward gaze.
• Postural Adjustments: If a patient exhibits a natural head-back posture (common in shorter individuals), the segment height should be lowered by 1 to 2 mm to prevent them from looking through the reading zone while walking. If the patient has a head-down posture or is tall, the segment height may be raised slightly.

Comparison Table of Segment Types