Wrong PD on Glasses: Symptoms and Tolerance
With a wrong PD, each lens centre sits away from your pupil. With stronger lenses, a few millimetres off can bring eye strain, headaches or a pulling feeling; with mild prescriptions it rarely matters.
- 2.5 mmANSI's PD limit for lenses stronger than 2.75 dioptres
- 0.67prism dioptres permitted on weaker lenses
- 1 mmhow closely two readings at home should match
Signs Your PD Is Wrong
Within a day or two of putting on new glasses, most people notice it. The usual signs are:
- sore or tired eyes after reading or looking at a screen;
- headaches that lessen after you remove the glasses;
- a sense that your eyes are being pulled, or a slight double image;
- sharp vision only when you look slightly to one side.
A new prescription your eyes are still adjusting to can produce the same signs. Wear new glasses for a few days, and if the feeling does not pass, have them checked.
How Accurate Does PD Need to Be?
ANSI Z80.1, the US lens standard, sets the limit according to lens power. At 2.75 dioptres or below, no more than 0.67 prism dioptres of unwanted prism is allowed. Once lens power goes past 2.75 dioptres, the centres of the lenses have to sit within 2.5 mm of the PD recorded on the order (ANSI Z80.1-2015). On bifocals, the near PD and the reading segments may differ by no more than 2.5 mm.
Put simply, a stronger lens leaves less room for error.
PD Off by 1 mm or 2 mm
Prentice's rule explains what a wrong PD does: the unwanted prism is the error in centimetres multiplied by the lens power (Prentice rule). An error of 2 mm equals 0.2 cm.
- On a -2.00 lens, multiplying 0.2 by 2 gives 0.4 prism dioptres, a figure that stays inside the limit.
- A -4.00 lens produces 0.8 prism dioptres, from 0.2 times 4. That exceeds what weak lenses allow, yet it still meets the 2.5 mm rule that covers strong lenses.
With a mild prescription, a 1 mm error is rarely noticed, since it works out to half that size.
What to Do About an Incorrect PD on Glasses
Start by measuring once more. Take two readings using the camera measurement or a ruler, and check them against the PD on your order. Reach out to the shop if the order is wrong by over 2 mm, or if the glasses do not feel right after a few days. Most shops will replace lenses that were made with a wrong PD. You can also have an optician mark the lens centres and compare them with your pupils.
Wrong PD Questions
Does PD really matter when you buy glasses?
Yes. It lines up each lens centre with your pupil. The stronger your prescription, the more PD matters.
Could a wrong PD cause damage to my eyes?
No lasting harm has been found. It brings on strain and discomfort, which clear up after the glasses are fixed.
Does a 2 mm PD difference count as large?
With mild lenses, it usually does not. A 2 mm error with lenses stronger than about 4 dioptres can be noticeable.