Single Vision vs Multifocal Lenses for Paraoptometrics: Choosing the Right Design

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Spectacle lens designs range from simple single vision lenses that correct one focal distance to multifocal designs that incorporate multiple powers for distance, intermediate, and near vision. Choosing the right design depends on the patient's age, refractive error, visual demands, lifestyle, and personal preferences. Paraoptometrics assist in gathering the information needed for this decision, explain the options to patients, take the fitting measurements needed for each design, and support patient adaptation after dispensing.

For the CPO and CPOA exams, you should understand what each lens design is, who it is appropriate for, the key fitting measurements required, and the advantages and limitations of each type. This knowledge also supports accurate documentation of lens orders.

Lens Design Overview

Single Vision — One focal point throughout the entire lens. Used for distance-only, near-only, or occupational-specific viewing. No age restriction. Required measurement: binocular or monocular PD, optical center height.
Flat-Top 28 Bifocal (FT-28) — Distance zone on top, 28mm flat-top D-shaped near segment on bottom. Visible dividing line. Wide reading area. Most common lined bifocal design. Required extra measurement: segment height.
Flat-Top 35 Bifocal (FT-35) — Same as FT-28 but with a wider 35mm segment for patients who need more reading width. Useful for musicians (sheet music), wide newspaper readers, and crafters.
Executive (Franklin) Bifocal — Segment spans the full width of the lens. Maximum near width. Heavier and more visible dividing line. Preferred by some occupational users who need wide, uninterrupted near vision at a specific distance (e.g., watchmakers, jewelers).
Flat-Top 7×28 Trifocal — Three zones: distance, 7mm intermediate band (≈ half the add), full add at bottom. Corrects intermediate distance (~60-70cm, computer range). Good alternative to progressives for computer-heavy presbyopes who prefer lined lenses.
Progressive Addition Lens (PAL) — Seamless progression from distance to near. No visible lines. Cosmetically preferred. Requires monocular PDs and fitting height. More complex fitting and longer patient adaptation. Peripheral blur zones are inherent.

Lens Order Terminology

OD / OS / OU — OD = right eye (oculus dexter), OS = left eye (oculus sinister), OU = both eyes (oculi uterque).
Sphere (Sph) — The base lens power in diopters. Negative = myopia. Positive = hyperopia. Plano (pl or 0.00) = no sphere power.
Cylinder (Cyl) — The power for astigmatism correction. Can be written in plus or minus cylinder form. Axis direction of cylinder.
Add — The additional plus power added to the distance prescription for near vision. Always positive. Ranges from +0.75 to +3.50D.
PD — Pupillary distance. Can be binocular (total, e.g., 64mm) or monocular (right/left separately, e.g., 32/32). PALs require monocular.
Seg Height — Distance from the bottom of the frame to the bifocal or trifocal dividing line. Measured with frame on patient.

Practice lens design questions for your exam

Opterio covers spectacle lens designs, fitting measurements, and optics with AI-powered explanations for CPO and CPOA.

Frequently Asked Questions

What is a single vision lens and who typically wears them?

A single vision lens has one focal point — the same prescription throughout the entire lens. This is the simplest lens design. Single vision lenses are appropriate for: (1) Pre-presbyopic patients — children and young adults who can accommodate freely and only need correction for distance or near, not both; (2) Full-time distance wearers (myopes who only need glasses for distance); (3) Dedicated reading glasses for presbyopic patients who only need near correction; (4) Occupational single vision for specific viewing distances (e.g., computer glasses set for 60cm working distance). Single vision lenses provide a large, unobstructed field of clear vision for their intended purpose. They have no peripheral distortion and require no adaptation.

What are the different types of bifocal segments and when is each used?

Bifocal segments are the near-vision additions applied to lined bifocal lenses. The most common types: (1) Flat-top 28 (FT-28) — the standard "D-segment" with a 28mm wide flat top; the most common bifocal in the United States. Provides a clear, wide reading area. (2) Flat-top 35 (FT-35) — wider 35mm segment for patients who need more reading width (wide newspaper or sheet music readers). (3) Round segment (22 or 25mm) — circular segment. Less commonly used today; was popular before flat-tops became standard. Cosmetically less visible but narrower reading area. (4) Executive (full-width) — the segment runs the full width of the lens at the bifocal line. Provides maximum near width; useful for occupational tasks requiring wide near vision. Heavier and cosmetically more visible. (5) Blended bifocal — the dividing line is blended/polished to be less visible cosmetically, but the optical performance is the same as a lined bifocal.

What is the difference between bifocal and trifocal lenses?

Bifocal lenses have two distinct zones: distance (top) and near (bottom add segment), separated by a visible dividing line. They work well for patients who primarily need distance and reading correction but create an image jump when the eye crosses the dividing line and provide no correction for intermediate distances (~50-70cm, computer range). Trifocal lenses add a third, intermediate zone — a band at the top of the segment that provides approximately half the add power, correcting for arm-length intermediate distances before transitioning to the full add at the bottom. Common trifocal design: flat-top 7x28 (7mm intermediate band) or flat-top 8x35. Trifocals are beneficial for patients who spend significant time at computer distance and prefer lined lenses over progressives. They are less popular than progressives for cosmetic reasons but are preferred by some patients who never adapt well to progressive peripheral distortion.

What fitting measurement is required for bifocal/trifocal lenses?

The critical additional measurement for lined bifocal/trifocal lenses is the segment height (seg height). Unlike progressives that use a "fitting height" at the pupil, bifocal seg height is measured from the bottom of the frame to where the dividing line of the segment should fall. The traditional fitting guideline is to position the bifocal dividing line at the lower eyelid margin when the patient looks straight ahead in primary gaze — this ensures the segment is readily accessible for near vision without being in the way for distance viewing. For trifocals, the dividing line of the intermediate zone is positioned at the lower pupil margin. Seg height is measured with the chosen frame on the patient in their natural head position. Unlike progressives, bifocals are somewhat more forgiving of small fitting errors due to the wider near zone.

How do you help a patient decide between progressives and bifocals?

This is a counseling conversation that paraoptometrics may support with factual information (while the final recommendation comes from the doctor). Key considerations: Patient lifestyle: active patients, computer users, professionals who need wide intermediate vision may not tolerate progressive limitations. Patient priorities: cosmetic preference (no visible line) → progressives; optical clarity and simplicity → bifocals/trifocals. Prior lens history: patients who have successfully worn progressives before will likely succeed again; patients who have failed progressives multiple times may do better with bifocals. Occupation: some jobs (mechanics, assembly workers, dental hygienists) benefit from the wider clear intermediate zone of a trifocal. Add power: very high adds (+3.00+) produce narrow progressive corridors; lined multifocals may provide better functional near vision. Patient age: younger presbyopes (+0.75 to +1.50 add) adapt easily to progressives; older patients starting progressives for the first time may have more difficulty.

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