The PD Journal
How Much PD Error Is Too Much? Prentice's Rule Explained
Last updated: October 2026 | Reading time: 8 minutes
Short answer: the harm from a PD error grows with lens power. Prentice's rule says the unwanted prism your eye sees equals the lens power in dioptres times the distance, in centimetres, between your pupil and the lens's optical centre. A 1 mm error is almost harmless in a weak lens and noticeable in a strong one. That is why weak single-vision glasses are forgiving and strong or progressive lenses are not.
Prentice's rule in one line
Prism (Δ) = lens power (D) × decentration (cm)
If the optical centre of a lens sits 1 mm (0.1 cm) away from your pupil, and the lens is 4.00 D, you look through 4 × 0.1 = 0.4 prism dioptres of unintended prism. Your eyes have to work to fuse the two images; the larger the prism, the more likely you are to feel strain.
Per-eye error, not total error
If your PD is entered 2 mm too wide and your face is symmetrical, each lens is off by 1 mm. So a 2 mm PD error is a 1 mm decentration per eye.
Prism caused by decentration (calculated)
| Lens power | 0.5 mm off | 1 mm off | 1.5 mm off | 2 mm off |
|---|---|---|---|---|
| 1.00 D | 0.05 Δ | 0.10 Δ | 0.15 Δ | 0.20 Δ |
| 2.00 D | 0.10 Δ | 0.20 Δ | 0.30 Δ | 0.40 Δ |
| 3.00 D | 0.15 Δ | 0.30 Δ | 0.45 Δ | 0.60 Δ |
| 4.00 D | 0.20 Δ | 0.40 Δ | 0.60 Δ | 0.80 Δ |
| 6.00 D | 0.30 Δ | 0.60 Δ | 0.90 Δ | 1.20 Δ |
| 8.00 D | 0.40 Δ | 0.80 Δ | 1.20 Δ | 1.60 Δ |
Values are power × decentration in cm. The rule applies along the direction of the offset, which for a PD error is horizontal, so use the lens power in the horizontal direction. With astigmatism, your optician can work out the exact figure.
What this means in practice
- Low powers (under about 2 D): a PD that is a millimetre or two out produces very little prism. Many people never notice.
- Moderate powers (3 to 4 D): accuracy starts to matter. Aim to be within about a millimetre.
- High powers (6 D and above): small errors create meaningful prism. Have a professional measure you. See PD for high prescriptions.
- Progressive lenses: the useful zones are narrow, so a misplaced lens costs you reading or distance clarity as well. See PD for progressive lenses.
We deliberately do not quote a single "acceptable error" here: tolerances depend on the standard, the prescription and the lens type, and your optician or the relevant standard is the right authority. If you want to learn what the tolerance table says, ask your optician for the current version of the lens-tolerance standard used in your country.
How accurate is the tool compared with these numbers?
Our tool shows an estimated error in millimetres with every result. Compare that figure with the table: a ±1 mm estimate on a 2 D lens is about 0.1 to 0.2 Δ per eye. The same estimate on an 8 D lens is around 0.4 to 0.8 Δ. See what limits online PD accuracy.
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Why PD matters for your glasses · PD and eye strain headaches · Free PD tool