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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.
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).
Measures each eye independently from the center of the nose bridge to the center of each pupil.
Total distance from the center of one pupil to the center of the other. A single number.
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.
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.
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.
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.
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.
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.
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.
| 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 | |
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.
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.
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.
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.
Understand Rx notation and how PD fits into the dispensing process.
Why accurate monocular PD is critical for progressive lens success.
Verify optical center placement matches the measured PD.
Browse all CPO and CPOA study topics organized by category.
Binocular PD is the total distance between the centers of both pupils, measured from the center of one pupil to the center of the other. Monocular PD measures each eye separately -- from the center of the bridge of the nose to the center of each pupil. Monocular PD is more accurate because most people's faces are not perfectly symmetrical, meaning the nose is not exactly centered between the eyes. The sum of the two monocular PDs equals the binocular PD. For example, monocular PDs of 31 mm (OD) and 32 mm (OS) give a binocular PD of 63 mm.
The optical center of each lens must align with the patient's pupil for clear, comfortable vision. If the PD is wrong, the optical centers are misplaced, effectively introducing unwanted prismatic effect (per Prentice's Rule). A PD error of just 2 mm in higher-power lenses can cause noticeable symptoms including eye strain, headaches, and blurred vision. The higher the lens power, the more critical the PD accuracy becomes.
Distance PD is measured with the patient looking at a distant target -- the eyes are essentially parallel. Near PD is measured with the patient looking at a near target (typically 40 cm) -- the eyes converge inward, making the near PD approximately 3-4 mm smaller than the distance PD. Near PD is used for reading glasses or the near portion of bifocals and progressives. If only one PD is provided, it is the distance PD.
Adult PD values typically range from about 54 to 74 mm, with the average being approximately 63 mm for women and 65 mm for men. Children have smaller PDs that increase as the face grows, generally ranging from 43 to 58 mm depending on age. PDs outside the normal range are not necessarily abnormal -- they simply reflect facial structure variation -- but unusually narrow or wide PDs should be double-checked for measurement accuracy.
PD is typically recorded in one of several formats. Binocular: "PD: 63" (distance only) or "PD: 63/60" (distance/near). Monocular: "PD: 31/32" meaning 31 mm OD and 32 mm OS for distance, or "PD: 31/32 (dist), 29.5/30.5 (near)." Always specify whether the value is distance or near if only one is given. When writing monocular values, the right eye is listed first, consistent with optometric convention.
Progressive lenses have a narrow corridor of clear vision that runs from the distance zone through the intermediate zone to the near zone. If the optical center is even slightly off from the pupil position, the usable corridor narrows and the patient experiences more peripheral distortion. Because progressives are so sensitive to alignment, monocular PD is strongly recommended over binocular PD to ensure each lens is precisely centered for each eye independently.