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Pupillary Distance (PD) and Why It Matters for Glasses

Edmonton Eye Doctors

13-03-2026

When ordering prescription glasses, one measurement plays a critical role in how clearly and comfortably you see: pupillary distance (PD). Many people assume PD is a number included in a prescription, but in reality it is a precision measurement used when crafting glasses. An accurate PD measurement helps ensure that the optical centre of your lenses aligns perfectly with your eyes, allowing your glasses to deliver the vision correction your prescription is designed to provide.

What Is Pupillary Distance?

The Distance Between Pupil Centres

Pupillary distance (PD) is the measurement, in millimetres, between the centres of your pupils. This measurement indicates to the lab optician where to position the optical centre of each lens so that light passes through the correct part of the lens when you look straight ahead. When the optical centre aligns with your pupils:

• Vision is clearer
• Eye strain is reduced
• Lenses perform as intended
• Glasses feel more comfortable to wear

Even small errors in PD can affect visual clarity.

How Pupillary Distance Is Measured

Precision Instrumentation

Traditionally, PD was measured with a pupil ruler. Today, many modern optical clinics use digital measurement systems that can capture highly accurate facial measurements. At a professional optical dispensary, PD is typically measured using:

• A digital optical measuring device
• A pupilometer
• A calibrated optical ruler

Digital measurement systems can also capture additional parameters that help ensure lenses are perfectly aligned within your chosen frame.

Why Pupillary Distance Should Be Measured for Each Eye

Monocular PDs are Best

Many prescriptions list PD as a single number, known as a binocular PD. For example, a PD of 62 mm. However, most people are not perfectly symmetrical. One eye is often slightly closer to the nose than the other. Because of this, opticians frequently measure monocular PD, which separates the measurement into two values:

• Right eye PD
• Left eye PD

For example:

• Right PD: 31 mm
• Left PD: 33 mm

Using monocular PD ensures that each lens is centred precisely for each eye.

Why PD Is Different for Distance and Near Vision

Eyes Move Inward To See Near Objects

Pupillary distance is not always the same for all types of lenses. When we focus on near objects, our eyes naturally converge inward. This means the distance between the pupils becomes slightly smaller. Because of this, PD measurements differ depending on the intended use of the glasses:

• Distance PD: Used for driving and general vision correction
• Near PD: Used for reading glasses or computer lenses

For progressive lenses, precise measurements are especially important to ensure smooth transitions between distance, intermediate, and near vision zones.

Why Measuring PD Yourself Is Very Difficult

Small Errors = Big Problems

Online guides often suggest measuring PD at home using a mirror or smartphone app. Unfortunately, these methods are rarely accurate enough for prescription eyewear. Self-measurement is difficult because:

• Your eyes naturally move when trying to measure them
• Holding a ruler precisely aligned is challenging
• Small errors of even 1–2 mm can affect lens performance
• Monocular PD cannot be reliably measured alone

Professional optical measurements ensure far greater accuracy.

Why Pupillary Distance Should Be Measured With the Frame On

PD Changes With Frame Size/Fit

One of the most overlooked aspects of PD measurement is that the frame itself affects where the lenses sit relative to your eyes. Different frames have different:

• Bridge widths
• Frame shapes
• Lens sizes
• How they sit on the nose

Because of this, PD should ideally be measured with the chosen frame on your face. This ensures the optical centre of the lenses is placed exactly where your eyes sit within that specific frame. Changing frames can slightly alter how lenses should be positioned.

Why PD Alone Is Not Enough

Optical Centre Requires 2 Measurements

Many people assume that once they know their PD, they can order glasses anywhere. In reality, PD alone is only part of the optical measurements needed to make high-quality glasses. Another important measurement is the vertical pupil height, which determines where the optical centre sits vertically in the frame. Without this measurement, the lenses may not align correctly when the glasses are worn. This is especially important for:

• Progressive lenses
• Bifocals
• High prescriptions
• Large frames

Professional optical measurements capture both horizontal and vertical alignment.

Why Pupillary Distance Is Not Included An Eye Exam

PD Is Measured Only To Craft Eyewear

Many patients are surprised that PD is not always listed on their prescription after an eye exam. The reason is simple: PD is not actually part of the medical eye exam. An eye exam focuses on evaluating:

• Eye health
• Vision clarity
• Prescription strength
• Detection of eye disease

PD, however, is part of the eyeglass construction process. Because PD depends on the specific frame and lens design, it is only measured when glasses are being purchased.

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