# Bifocal Lens Designs > Bifocal designs for Optometry: executive, round and flat-top segments, image jump computation, slab-off prism and segment height fitting rules. - Canonical URL: https://prepelephant.com/topics/allied/optometry/bifocal-designs - Exam / course: Allied Health · Subject: Optometry - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Bifocal Lens Designs", PrepElephant, https://prepelephant.com/topics/allied/optometry/bifocal-designs ## Direct answer Every bifocal bonds a near segment to a distance lens, and the design decides its physics: the Executive (one-piece, full-width) places the segment's optical centre on the dividing line, abolishing vertical image jump; the round segment (24-25 mm) hangs its centre 5 mm or more below the line, jumping most; the flat-top or D-segment — 22 to 45 mm, with 28 the workhorse — trades a modest jump for a wide reading field; blended designs hide the line inside a smeared 2-3 mm transition. Image jump is Prentice arithmetic — addition times the vertical distance between the distance centre and the segment centre — and anisometropes, whose two lenses disagree at the reading position, need slab-off or dissimilar segments. Segment height set at the lower pupil margin finishes the job. ## What you must remember - **Design ladder:** Executive — full width, segment centre on the line, no vertical jump; round seg (24-25 mm) — largest jump; curved-top and flat-top/D (22-45 mm); blended — no visible line, 2-3 mm of blur. - **Image jump formula:** jump (Δ) = addition × distance in centimetres from segment line to segment optical centre — a +2.00 add with its centre 5 mm below the line jumps 1 Δ base-down. - **Reading-position prism:** Prentice at the visual point for the whole lens; vertical imbalance beyond about 1.5 Δ between the eyes is symptomatic. - **Slab-off:** base-up prism bi-centically ground into the more minus (or less plus) lens; reverse slab-off is pre-cast base-down in the more plus lens; dissimilar segments are the third route. - **Segment height convention:** top of segment at the lower pupil margin (roughly lower limbus) in primary gaze, measured in the fitted frame; general-purpose FT 28 sits about 22-24 mm. - **Occupational tailoring:** FT 35 or Executive for sustained full-width near work; trifocals carry an intermediate of about 50 per cent of the add, top 3-5 mm below the distance centre. - **Prentice worked example:** a −6.00 D lens read 8 mm below its centre gives 4.8 Δ base-down — beside a plano fellow eye, that is slab-off territory. - **Dispensing checks:** seg heights level, seg centres inset 2-2.5 mm per eye for convergence, near optical centres on the reading axes. ## The anisometropic bifocal, computed A 62-year-old, right −5.00 DS, left plano, addition +2.00, reads 10 mm below the distance centres. At the near visual point the right lens carries −3.00 D effective (−5.00 + 2.00): 3 Δ base-down by Prentice; the left segment alone gives 2 Δ base-up. The images shear nearly 5 Δ apart vertically, and the patient returns within a week saying the print splits. Options: slab-off 3-4 Δ base-up ground into the right lens; dissimilar segments; a progressive, which smears rather than steps the imbalance; or contact lenses, which sidestep spectacle prism entirely. Contrast the routine case: a presbyope with −0.50 each eye and a +2.00 add takes a flat-top 28 — jump = 2.00 × 0.5 = 1 Δ, a barely perceptible bob tolerated within days. The same arithmetic explains the satisfied routine patient and the furious anisometrope, and answers the examiner's question of which lens receives slab-off, in which direction. ## Where marks are lost Direction errors dominate: slab-off is base-up on the more minus lens, and candidates reverse either half as often as not. Jump versus displacement: jump is the sudden shift crossing the segment line; displacement at a stationary reading point is Prentice at the visual point — two sums, one rule. The Executive's no-jump property scores only when explained (centre on the line), not asserted. And seg heights measured on the trial frame, or guessed from an old pair, lose the patient: the line must sit relative to this pupil, in this frame, as worn. ## Frequently asked questions ### Which bifocal design eliminates vertical image jump and why? The Executive, because its segment optical centre lies on the dividing line, so no prismatic step occurs as the pupil crosses. ### How is image jump calculated? By Prentice's rule on the addition: jump in prism dioptres equals add power times the distance in centimetres from segment line to segment centre. ### What is slab-off prism and which lens receives it? Base-up prism bi-centically ground into the more minus (or less plus) lens to cancel base-down vertical imbalance at near in anisometropes. ### Where is the bifocal segment line fitted? Top of the segment at the lower pupil margin in primary gaze, measured in the fitted frame, with 2-2.5 mm per-eye inset for convergence. ### What intermediate power does a trifocal carry? About half the near addition, its top 3-5 mm below the distance optical centre, serving arm's-length tasks.