Spectacle Dispensing Reference: PD, Fitting Height, Tolerances
Last updated: October 2026
Measurements, fitting and quality checks for spectacles, and what to check when a patient says "my new glasses don't feel right".
Measurements
- Pupillary distance (PD): measure monocular PDs for each eye (pupil centre to bridge of nose) with a pupillometer or ruler; distance PD in adults is typically 54–74 mm. Near PD is about 3–4 mm less than distance PD.
- Fitting height: for progressives, measure from the centre of the pupil to the lowest point of the lens rim in the chosen frame, with the frame adjusted and the patient looking straight ahead. Most progressive designs need at least 14–18 mm.
- Bifocal segment height: usually at the lower lid margin (lower for drivers, higher for desk work).
- Vertex distance: record it for powers above ±4.00 D and if the frame sits differently from the trial frame. See the transposition and vertex tool.
- Pantoscopic tilt and wrap: 8–12° tilt is typical; lower the optical centre about 1 mm for every 2° of tilt.
Tolerances for finished lenses (ANSI Z80.1)
| Item | Tolerance |
|---|---|
| Sphere power, up to ±6.50 D | ±0.13 D |
| Sphere power, above ±6.50 D | ±2% |
| Cylinder power, up to 2.00 D | ±0.13 D |
| Cylinder power, 2.00–4.50 D | ±0.15 D |
| Cylinder power, above 4.50 D | ±4% |
| Cylinder axis: 0.25 D cylinder or less | ±14° |
| Axis: above 0.25 to 0.50 D | ±7° |
| Axis: above 0.50 to 0.75 D | ±5° |
| Axis: above 0.75 to 1.50 D | ±3° |
| Axis: above 1.50 D | ±2° |
| Add power, up to +4.00 D | ±0.12 D |
| Unwanted horizontal prism (pair) | up to 0.67 Δ (or PD within ±2.5 mm) |
| Unwanted vertical prism imbalance | up to 0.33 Δ (or optical centres within 1 mm) |
ISO 21987 sets similar tolerances. Check any lens that fails with a focimeter before blaming the patient.
"My new glasses don't feel right": checklist
- Confirm the lenses match the prescription (power, axis, add) on a focimeter.
- Check PDs and optical centres; for high powers, small errors cause prism (Prentice's rule).
- Check fitting height and frame adjustment for progressives; a dropped frame is a common cause of reading problems.
- Compare with the old glasses: large changes in cylinder axis, base curve or lens material often cause temporary distortion; changes of more than 1.00 D of cylinder or an axis shift in older adults may need a partial prescription.
- Check for anisometropia (more than 2–3 D difference causes aniseikonia) and vertical prism at near in bifocal wearers (consider slab-off).
- Give 1–2 weeks to adapt to progressives or a new cylinder before remaking, unless something is clearly wrong.
- If everything checks out, re-refract: the prescription itself may be the problem.
Related: Prentice's rule, vertex distance, lens coatings guide, reading a prescription (for patients).
Key sources
- ANSI Z80.1-2020. Prescription Ophthalmic Lenses: Recommendations
- ISO 21987:2017. Ophthalmic optics: mounted spectacle lenses
- Jalie M. Ophthalmic Lenses and Dispensing. 3rd ed. Butterworth-Heinemann; 2008
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