How Important is Pupillary Distance (PD)?

What is PD (Pupillary Distance)?

Pupillary Distance (PD), is the measurement of distance between your pupils. PD plays a varying role in eyewear fitting depending on the strength of your prescription and the size of your frames.  When selecting a pair of glasses matching your PD to the frames ensures that the lenses are centered correctly in front of your eyes. 

How is PD used?

PD, or Pupillary Distance is used to position the optical centers of your lenses (the spot of a lens where you can see the clearest) directly in front of your pupils. Having an accurate PD ensures that you’ll be able to see clearly and comfortably through your glasses, as the lenses will work with the natural way your eyes focus.

The Impact of PD for Different Prescriptions

PD has less of an impact with low prescriptions

For single vision lenses with low power (less than +/- 1.50 diopters), the PD is not a critical factor. In such cases, slight deviations in PD measurement have a negligible effect on your vision or comfort. You’ll still want your PD to be accurate, as missaligned lenses in some cases can lead to eye strain and headaches.

PD is more Important with Higher Prescriptions

In single vision lenses, as the lens power exceeds +/- 1.50 diopters, the accuracy of PD becomes important. Incorrect PD measurements in higher prescriptions can cause discomfort, eyestrain, headaches or even blurred vision, as the lenses may not align correctly with your line of sight. If your prescription exceeds +/- 3.50 a missaligned PD can cause thicker lenses depending on the severity of the alignment.  

PD is Crucial for Progressive Lenses

If you require progressive lenses an accurate PD is essential. These lenses are designed to provide seamless vision correction for multiple distances, near, intermediate, and far. Having an accurate PD ensures that the different vision zones in the lenses are properly aligned with your eyes, allowing you to transition smoothly between viewing distances without distortion.

Does PD Change?

For most adults, once their PD is measured accurately, it does not change significantly over time. PD can change during childhood as part of the body’s natural growth process. But once an individual reaches adulthood, PD stabilizes and remains constant. Exceptions to this are rare and usually result from specific medical conditions or injuries that affect the eye structure.

How to Measure Your Own PD

Before you start measuring your PD, it’s important to know your prescription, if you have a prescription stronger than +/- 1.50, or plan to get Bifocal, Trifocal or Progressive Lenses, it’s recommended that you go and see an Optician. Measuring an inaccurate PD for your glasses can cause discomfort, eyestrain, headaches and more. 

What you’ll need

• A straight-edge ruler with millimeter measurements
• A mirror, mounted at eye level
• A pen or pencil and some paper

Step 1: Prepare Your Setup

Position yourself comfortably about 8 inches away from a mirror, ensuring the mirror’s surface is level with your eyes to maintain accuracy throughout the process.

Step 2: Align and Measure

• Place the ruler horizontally across your eyebrows or just below your eye line.
• Start by aligning the zero end of the ruler with the pupil center of one eye.
• Without shifting your head or the ruler, note the measurement on the ruler that aligns with the center of the other pupil.

Step 3: Record Your PD

The millimeter reading where the second pupil aligns on the ruler is your PD. Adult PD usually falls between 54 mm and 74 mm. Write down this number.

Step 4: Retake Your PD

Repeat the process several times. This is to help ensure you’re taking an accurate measurement. A few millimeters of variance can be expected, but larger discrepancies suggest the need for more careful measurement.

Additional Tips for Reliable Measurement:

• Conduct the measurement in a well-lit environment to ensure your pupils are clearly visible.
• Keep both eyes open during the measurement to prevent any skew caused by closing one eye.
• For those struggling with the method above, consider measuring each eye separately from the center of the nose bridge to the pupil center, and then add these measurements together.

Do PD Measuring Apps Work?

Digital apps for measuring Pupillary Distance (PD) offer a convenient alternative to traditional methods. While some of these apps are fairly advanced and can provide reasonably accurate results, the accuracy and effectiveness can vary significantly from one app to another. Factors such as the quality of the smartphone camera and the lighting conditions,  can all impact the precision of the measurement.

It’s important to note that while these apps are suitable for general use, they may not always deliver perfect accuracy. If you have a prescription over +/- 1.50, even small inaccuracies in PD measurement can result in poor visual comfort or eye strain. Making it important to visit an optician for a professional measurement.

Accurately measuring your Pupillary Distance (PD) is important to make sure you can see clearly and comfortably. Minor inaccuracies in PD may be tolerable for low prescriptions, but if you have a higher prescription or progressive lenses you’ll want to have your PD measured professionally. Especially when dealing with complex prescriptions. 

About The Author:

Will Benjamin is an advocate for independent eyewear and one of the driving forces behind Project Spex. With a passion for unique, collectible, and limited-edition eyewear, Will aims to inspire people to build their own collections through Project Spex, while supporting the success of independent opticals.