Spectacle Lens Designs: Aspheric, Bifocals and Progressive Lenses (Fitting and Troubleshooting)

Last updated: November 2026

RPC Rounds for Optometrists · B.Optom study guide

The same prescription can be made in many designs. Single-vision lenses are made flatter and thinner with aspheric surfaces; presbyopes can choose bifocals, trifocals, occupational lenses or progressives. Each has trade-offs that the dispenser must explain and fit correctly.

Single-vision designs

• Best-form (corrected curve) lenses choose a base curve that minimises oblique astigmatism (Tscherning ellipse). Deeper curves are optically better but bulkier.
• Aspheric lenses flatten the curve towards the edge: thinner, lighter, less magnification or minification, better appearance, with good off-axis optics. They must be centred accurately on the pupil (monocular PD and height).
• Lenticular designs for very high powers: only a central zone carries the full power.

Bifocals and trifocals

• Image jump: the sudden prism at the segment top (Prentice: distance from segment top to segment optical centre × add). Flat-tops minimise it.
• Fitting height: segment top usually at the lower lid margin (or slightly lower for distance-heavy users, higher for near-heavy users).

Progressive addition lenses (PALs)

• Power increases smoothly from the distance zone through a corridor to the near zone, with unavoidable areas of unwanted astigmatism on each side (Minkwitz theorem: the faster the power change, the more unwanted astigmatism).
• Hard designs: wide clear zones with sharp, strong distortion at the edges. Soft designs: narrower zones with gentler, spread-out distortion, easier adaptation.
• Corridor length: short corridors (about 11–14 mm) for shallow frames; standard about 15–18 mm. Free-form and personalised designs allow for frame wrap, tilt and vertex.
• Occupational (office) lenses: degressive designs for intermediate and near, wider zones for computer work.

Measuring and fitting a progressive

1. Choose a frame with enough depth for the corridor and near zone (usually at least 14–18 mm from the pupil centre to the bottom of the lens, depending on the design) and adjust it on the face first: pantoscopic tilt about 8–12°, close vertex, correct face form.
2. Measure monocular PDs from the bridge (pupil reflex) and fitting heights to the pupil centre, with the patient looking straight ahead at your eyes at the same level.
3. Mark the pupil centres on the dummy lenses for accuracy.
4. On delivery, check the fitting cross is at the pupil centre, distance and near vision, and that the patient can find the near zone by lowering the eyes, not the head.

Common progressive complaints

See spectacle non-tolerance troubleshooting for a full checklist.

Test yourself: flip cards

Quick quiz

For standard, short-corridor, wide, extra-wide and personalised progressives compared with a diagram, see progressive lens types. For daily counter use, see the lens types comparison and order checklist and the sunglasses, polarised and photochromic lens guide.

Related: Spectacle Dispensing Reference: PD, Fitting Height, Tolerances, Frame Selection and Spectacle Fitting: Boxing System, PD, Fitting Height, Tilt and Wrap, Lensometer (Focimeter) Step by Step: Single Vision, Prism, Bifocals and Progressive Lenses, Spectacle Non-Tolerance: A Step-by-Step Troubleshooting Guide for Optometrists, Refraction Calculators: Near Add, AC/A Ratio, Retinoscopy and Prism. Back to the optometry curriculum.

Key sources

• Jalie M. Ophthalmic Lenses and Dispensing. 3rd ed. Butterworth-Heinemann; 2008
• Meister DJ, Fisher SW. Progress in the spectacle correction of presbyopia. Clin Exp Optom. 2008;91:240-250 and 251-264
• Brooks CW, Borish IM. System for Ophthalmic Dispensing. 3rd ed. Butterworth-Heinemann; 2007