What is the difference between monocular and binocular PD?

Binocular PD is the total distance between both pupils as a single number (e.g., 64 mm). Monocular PD measures each eye's distance from the center of the nose individually, giving two values (e.g., OD: 31, OS: 33). Monocular PD is more accurate because it accounts for facial asymmetry and is preferred for progressive lenses and high prescriptions.

Why is near PD smaller than distance PD?

When the eyes converge (turn inward) to focus on a near object, the pupils move closer together. This convergence reduces the PD by approximately 3 to 4 mm compared to the distance PD. Near PD should be used for single-vision reading glasses to ensure the optical centers align with the converged pupil positions.

Which PD type should I use for progressive lenses?

Progressive lenses are typically fitted using monocular distance PD. The monocular values account for facial asymmetry, and the distance PD positions the fitting cross at the correct horizontal location for the distance viewing zone. The progressive lens design handles the convergence for near viewing through its corridor.

How much does PD change from distance to near?

Near PD is typically 3 to 4 mm less than distance PD. This reduction is caused by convergence of the eyes when focusing on a near target. The exact amount varies by individual and can be measured directly or estimated by subtracting 3 mm from the binocular distance PD.

PD Measurement Types: Monocular, Binocular, Distance, and Near PD Explained

Types of Pupillary Distance Measurements

Pupillary distance (PD) is the distance between the centers of the patient's pupils, used to align the optical centers of spectacle lenses with the patient's visual axes. There are several types of PD measurements, and choosing the correct type for each situation ensures optimal lens centering.

Monocular vs. Binocular PD

Binocular PD

Binocular PD is the total distance from the center of the right pupil to the center of the left pupil. It is a single number (e.g., 64 mm) and assumes symmetry between the two sides of the face.

Monocular PD

Monocular PD measures the distance from the center of each pupil to the center of the bridge of the nose individually. The result is two values: one for the right eye (OD) and one for the left eye (OS). For example, OD: 31 mm, OS: 33 mm (total binocular PD = 64 mm).

Type	Values	Accuracy	When to Use
Binocular	One number (e.g., 64)	Assumes symmetry	Simple single vision, when symmetry is confirmed
Monocular	Two numbers (e.g., 31/33)	Accounts for asymmetry	Progressive lenses, high Rx, precise centering

Most faces are not perfectly symmetrical. The nose may be slightly off-center, or one eye may be positioned differently from the other. Monocular PD measurements account for this asymmetry and should be used whenever possible, especially for progressive lenses and high prescriptions where centering errors are most noticeable.

Distance PD vs. Near PD

Distance PD

Distance PD is measured while the patient fixates on a distant target (optical infinity). The eyes are in a parallel position, and the PD is at its widest. Distance PD is used for:

Distance portion of multifocal lenses
Single-vision distance glasses
General-purpose eyewear

Near PD

Near PD is measured while the patient fixates on a near target (typically 40 cm). The eyes converge inward, bringing the pupils closer together. Near PD is typically 3 to 4 mm less than distance PD. Near PD is used for:

Single-vision reading glasses
Near segment centering in bifocals and trifocals

Using distance PD for single-vision reading glasses. If the lenses are intended exclusively for near work, the optical centers should be positioned at the near PD (closer together) to align with the converged eye position. Using distance PD for reading glasses causes the patient to look through a decentered point, inducing base-out prism.

Special PD Considerations

Progressive Lens PD

Progressive lenses are typically fitted using monocular distance PD for the fitting cross position. The progressive design itself handles the transition to near by narrowing the corridor slightly, so a separate near PD is not usually needed for the lens order. However, some premium progressive designs accept separate distance and near PD values for enhanced optimization.

PD for Prism Prescriptions

When prism is prescribed, the PD measurements still determine where the optical center should be placed. The prism reference point (PRP) replaces the optical center as the point where prism is verified, but the horizontal positioning still follows the PD.

Children's PD

Children have smaller PDs than adults (typically 40-55 mm for young children, growing to adult values in the mid-teens). Their PD should be measured at every visit since it changes with facial growth. Monocular PD is especially important for children because facial asymmetry is common.

Always record the type of PD you measured. A PD written as "64" could be binocular distance PD, binocular near PD, or the sum of monocular PDs. Be specific: "Distance PD: 32/32" or "Binocular Distance PD: 64 mm" eliminates ambiguity and prevents errors in the lab.

Clinical Relevance

PD accuracy directly affects the visual comfort and performance of spectacle lenses. Selecting the correct PD type for each situation, and recording it clearly, prevents dispensing errors that can cause patient complaints. Understanding the different types and when to use each is fundamental ABO exam knowledge.

Key Takeaways

Binocular PD is a single total measurement; monocular PD provides separate right and left values
Monocular PD is preferred for progressive lenses and high prescriptions
Distance PD is measured with the patient looking at a far target; near PD at a 40 cm target
Near PD is typically 3-4 mm less than distance PD due to convergence
Use distance PD for distance lenses and near PD for reading glasses
Always specify the type of PD recorded to prevent lab errors