How to Measure PD: Monocular, Binocular and Near PD

For distance PD with a millimeter ruler, sit at eye level about 40 cm away. Close your right eye, have the patient look at your left eye, and set zero on their right pupil center. Then switch eyes and read the scale at their left pupil center. Take monocular PDs for asymmetric faces or progressives.

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Why PD matters

Pupillary distance (PD) tells the lab where to put each optical center. If the centers don’t sit in front of the pupils, the wearer looks through an off-center point and gets prism they didn’t need (see prism base direction). In ABO-NCLE’s test specifications, taking measurements for a fit sits under Dispensing Procedures (20%), and the tools used to take them under Instrumentation (15%), checked Nov 2, 2026.

Ruler method: binocular distance PD

Sit squarely in front of the patient, about 40 cm away, with your eyes at the same height as theirs.
Rest the PD ruler across the bridge of the nose, tilted back into the recessed part where the frame will sit. Steady your hand lightly against the patient’s head.
Close your right eye. Ask the patient to look at your open left eye. Line zero up with the center of their right pupil.
Keep the ruler still. Close your left eye, open your right, and ask the patient to look at your right eye. Read the mark at the center of their left pupil. That is the binocular distance PD.
Close your right eye again and confirm zero hasn’t drifted.

If the pupil center is hard to see, use the same edge on both eyes (left edge to left edge), or the limbus on dark irises. Measuring inner edge to inner edge reads too small; outer to outer reads too large.

Pupilometer

A corneal reflex pupilometer rests on the nose like a frame. The patient fixates a light inside it, and you move a hairline onto each corneal reflection. It reads each eye separately, is less prone to parallax than a ruler, works if you have good vision in only one eye, and usually has a near setting. It locates the corneal reflex, not the exact line of sight, so its reading can differ slightly from a pupil-center measurement.

Monocular PDs

A monocular PD is the distance from one pupil center to the center of the nose (the bridge). Faces are rarely perfectly symmetric, so halving a binocular PD can put one optical center in the wrong place. Take monocular PDs for progressives and for strong or unequal prescriptions.

With a ruler: measure the binocular PD as above, note the reading at the center of the nose (the right monocular PD), then subtract it from the binocular reading for the left. Example: binocular 64, nose at 31, so R 31 and L 33.

Near PD

The eyes converge to read, so the near PD is smaller than the distance PD. To measure it with a ruler, sit at the patient’s reading distance, put your one open eye in front of their nose, have them look at it, and read right pupil center to left pupil center without switching eyes. Measure with the chosen frame on where you can.

To estimate it, use similar triangles from the eyes’ centers of rotation, about 27 mm behind the spectacle plane (roughly 13.5 mm from cornea to center of rotation plus the vertex distance):

near PD ≈ distance PD × l ÷ (l + 27), with l the reading distance in millimeters.

Example: 64 mm at 40 cm. 64 × 400 ÷ 427 = 59.95, so about 60 mm. Bifocal segment inset per lens is then (64 − 60) ÷ 2 = 2 mm.

From PD to decentration

Frame PD is A + DBL. Decentration per lens is (frame PD − PD) ÷ 2, or, with monocular PDs, half the frame PD minus each monocular PD.

A 52, DBL 18, PD 64: frame PD 70, so (70 − 64) ÷ 2 = 3 mm in per lens.
Same frame, monocular R 30 / L 34: half frame PD 35, so R 5 mm in, L 1 mm in.

Check these in the decentration calculator, then carry the decentration into minimum blank size.

Errors and checks

Parallax: your PD very different from the patient’s, or sitting closer than about 40 cm.
Eyes not level with the patient’s, or sitting off to one side.
The ruler not in the spectacle plane.
Head movement by you or the patient.
The patient not looking at your open eye, or one eye drifting. With a known eye turn, cover the eye you aren’t measuring.
Mixing edges: use the same side of each pupil or limbus.

Practice problems

Original problems.

A ruler reads 32 at the center of the nose and 63 at the left pupil center (zero on the right pupil center). Find the monocular PDs.
Show answer
Right = 32 mm. Left = 63 − 32 = 31 mm. Binocular PD = 63 mm.
Frame A 54, DBL 16. Binocular PD 62. Decentration per lens?
Show answer
Frame PD = 54 + 16 = 70. (70 − 62) ÷ 2 = 4 mm in per lens.
Same frame (A 54, DBL 16) with monocular PDs R 29, L 33.
Show answer
Half the frame PD = 35. R: 35 − 29 = 6 mm in; L: 35 − 33 = 2 mm in.
Estimate the monocular near PD for a 33 mm monocular distance PD at a 35 cm reading distance.
Show answer
33 × 350 ÷ (350 + 27) = 33 × 0.928 = 30.64 → about 30.6 mm.
A binocular distance PD of 64 mm and a measured near PD of 60 mm. Bifocal segment inset per lens?
Show answer
(64 − 60) ÷ 2 = 2 mm per lens.

Standard texts used for this page: Brooks CW, Borish IM. System for Ophthalmic Dispensing, 3rd ed. Butterworth-Heinemann, pp. 25–33 (ruler technique, errors, monocular and near PD, segment inset); Jalie M. The Fitting of Spectacle Lenses (ABDO article) (near centration by similar triangles); and the Community Eye Health Journal and ABDO articles linked under sources.

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Frequently asked questions

How do opticians measure PD with a ruler?

Rest the ruler on the bridge of the nose in the spectacle plane, at about 40 cm with your eyes level with the patient’s. Close your right eye, have the patient look at your open left eye, and line zero up with the center of their right pupil. Without moving the ruler, close your left eye, have them look at your right eye, and read the mark at the center of their left pupil. Switch back once to check that zero hasn’t moved.

When do I need monocular PDs instead of binocular?

When the two eyes are not the same distance from the center of the nose, or when the lens design needs each eye centered on its own: progressives in particular, and strong or unequal prescriptions, where a misplaced optical center causes unwanted prism. Halving a binocular PD assumes a symmetric face.

How do you estimate near PD from distance PD?

Measuring it is best: sit at the patient’s reading distance and have them look at your one open eye, with the frame on. To estimate it, use similar triangles. The centers of rotation sit about 27 mm behind the spectacle plane, so near PD ≈ distance PD × l ÷ (l + 27), with l the reading distance in mm. At 40 cm, a 64 mm PD gives about 60 mm.

Can a PD measurement be off by a millimeter?

Yes, easily, which is why technique matters. Standard texts list parallax as a main cause: if your own PD differs a lot from the patient’s, or you sit too close, the reading shifts. Uneven eye height, a moving head, a patient who doesn’t fixate your open eye, and measuring from inner or outer pupil edges all add error. Recheck zero before you read.

Is PD part of the ABO practical exam?

ABO-NCLE’s 2026 practical exam handbook doesn’t use the term PD in its test specifications, but its instrumentation domain covers measuring tools such as a pupilometer and a ruler, and its formula and fitting items include placing optical centers (checked Nov 2, 2026).

Source: ABO-NCLE — ABO Practical Examination, Oct–Dec 2026 (links the Practical Examination Handbook with the ABOP test specifications) (opens in a new tab)

Official sources

Last reviewed Nov 2, 2026. Requirements, fees, and dates change — always confirm with the official source before you register or apply. ABO-NCLE and IJCAHPO are the only authorities for their own exam facts, so those facts rest on a single source.