Pupillary distance (PD) is the distance between the centers of the pupils, and it is one of the most important measurements in ophthalmic dispensing. Every pair of glasses depends on the optical centers of the lenses being aligned with the patient's pupils. Get the PD wrong, and the patient looks through the wrong part of the lens, introducing unwanted prism that can cause eye strain, headaches, and blurred vision.
On the CPO and CPOA exams, PD measurement is tested as both a practical skill and a conceptual topic. You need to know how to measure PD with a ruler and with a pupillometer, the difference between monocular and binocular PD, why near PD differs from distance PD, and the consequences of an incorrect measurement. The exams also connect PD to optical concepts like Prentice's Rule and decentration.
This guide covers everything you need to know about PD measurement: what it is, why it matters, how to measure it both ways, average values, how to record it, common errors, and why monocular PD is essential for progressive lenses.
What Is Pupillary Distance?
Pupillary distance (PD) -- also called interpupillary distance (IPD) -- is the horizontal distance between the center of one pupil and the center of the other, measured in millimeters. When the patient looks at a distant target, the eyes are nearly parallel and the PD is at its maximum (distance PD). When the patient looks at a near target, the eyes converge inward and the effective PD decreases (near PD).
The reason PD matters is simple physics. Each ophthalmic lens has an optical center -- the point through which light passes without being deviated. For clear, comfortable vision, the optical center must align with the patient's pupil. If it does not, the lens acts like a prism at the point the patient looks through, bending light in an unintended direction. The stronger the lens, the more prism is induced by a given amount of decentration (this is Prentice's Rule: prism = decentration in cm x lens power in diopters).
Monocular vs Binocular PD
Monocular PD
Measures each eye independently from the center of the nose bridge to the center of each pupil.
- More accurate -- accounts for facial asymmetry
- Required for progressive lenses
- Recorded as OD/OS (e.g., 31/32)
- Recommended for all prescriptions
Binocular PD
Total distance from the center of one pupil to the center of the other. A single number.
- Faster to measure
- Acceptable for single vision lenses
- Recorded as single value (e.g., 63)
- Assumes symmetrical face
Exam Tip
Most people's noses are not perfectly centered between their eyes. The difference between right and left monocular PDs is commonly 1-2 mm. For a patient with a binocular PD of 64, the monocular values might be 31 OD and 33 OS. This asymmetry is why monocular PD is preferred -- especially for progressive and high-power lenses.
Distance PD vs Near PD
When you look at something far away, your eyes point nearly straight ahead -- parallel. When you look at something close, your eyes turn inward (converge) to focus on the near object. This convergence brings the pupils closer together, making the near PD smaller than the distance PD.
The difference is typically about 3-4 mm. If a patient's distance PD is 64 mm, their near PD would be approximately 60-61 mm. Near PD is used when ordering reading glasses (single vision near) or when the lab needs to position the near segment of a bifocal or the reading zone of a progressive. Distance PD is used for distance-only glasses and for positioning the distance optical center of multifocal lenses.
Measuring PD with a Ruler
Step 1: Positioning
Stand or sit directly in front of the patient at the same height, approximately 40 cm (16 inches) away. Your eyes should be at the same level as the patient's eyes. Hold the PD ruler (a standard millimeter ruler works) across the bridge of the patient's nose.
Step 2: Measure the Right Eye (Monocular)
Close your left eye and have the patient look at your open right eye. Align the zero mark of the ruler with the center of the patient's right pupil. Note the mark at the center of the bridge of the nose. This is the right monocular PD. Using your right eye eliminates parallax error for the right-side measurement.
Step 3: Measure the Left Eye (Monocular)
Now close your right eye and have the patient look at your open left eye. Without moving the ruler, read the measurement from the center of the nose bridge to the center of the patient's left pupil. This is the left monocular PD. Using your left eye eliminates parallax for the left-side measurement.
Step 4: Verify Binocular
Add the two monocular measurements together. This should equal the binocular PD. You can also measure binocular directly: have the patient fixate on a distant target, close your right eye, align zero with the patient's right pupil, then close your left eye and open your right eye to read the mark at the patient's left pupil center. Repeat to confirm.
Measuring PD with a Digital Pupillometer
A digital pupillometer is a handheld device specifically designed for PD measurement. It is more precise than a ruler (typically to 0.5 mm) and automatically measures monocular PD for each eye. The patient looks through the device at an internal target while the instrument detects the pupil positions.
To use a pupillometer: have the patient hold the device against their brow. Set it to distance or near mode as needed. Ask them to look at the target light. Press the measure button. The device displays both monocular and binocular PD values. Most pupillometers also allow you to switch between distance and near with a toggle, automatically adjusting the target position to induce the appropriate vergence.
Digital pupillometers remove most of the parallax and alignment issues of ruler measurement. They are the standard in most dispensaries and are preferred whenever available. However, the exam expects you to know the ruler technique as well, since it is the fallback when a device is unavailable.
Practice PD measurement and dispensing questions
Opterio covers ophthalmic optics, dispensing, and all CPO/CPOA exam domains.
Average PD Values
| Population | PD Range (mm) | Average (mm) |
|---|---|---|
| Adult women | 54-68 | ~62 |
| Adult men | 58-74 | ~65 |
| Children (ages 5-12) | 43-58 | ~50-54 |
| Near PD adjustment | Subtract 3-4 mm from distance PD | |
Common PD Measurement Errors
Parallax Error
If you are not directly in front of the patient, or if you use the same eye to sight both pupils, parallax will shift your readings. Always close one eye at a time as described in the ruler technique to eliminate this.
Patient Not Fixating Properly
If the patient looks at the ruler instead of at your eye (or at a distant target for distance PD), the measurement will be incorrect. For distance PD, having the patient fixate on a target at 6 meters (or your eye as a proxy) is essential.
Confusing Distance and Near PD
Using distance PD for reading glasses or near PD for distance glasses both cause decentration. Always confirm which PD the lab order requires and measure accordingly.
Using Only Binocular PD for Progressives
Progressive lenses require monocular PD to properly center the narrow viewing corridor for each eye. A binocular PD that is simply split in half (e.g., 64/2 = 32 each) ignores facial asymmetry and can cause adaptation problems.