10 PD Myths Debunked: What You've Heard About Pupillary Distance That Isn't True

PD Myths Debunked

Last updated: January 2025 | Reading time: 9 minutes

Pupillary distance seems straightforward—the distance between your pupils, measured in millimeters. Yet misconceptions about PD are surprisingly common, repeated on forums, by well-meaning friends, and sometimes even by optical professionals who should know better.

These myths can lead you to order wrong glasses, dismiss legitimate problems, or overpay for services you don't need. Let's separate fact from fiction.

Myth #1: "Everyone's PD Is Around 63mm"

The myth: PD doesn't vary much, so using 63mm (or 64mm) works for most people.

The reality: Adult PD ranges from approximately 54mm to 74mm—a 20mm spread. Using 63mm for someone whose actual PD is 57mm creates a 6mm error, potentially causing significant symptoms with moderate-to-strong prescriptions.

The "average" exists because it's mathematically inevitable when you calculate means. But no individual should use the average unless their actual PD happens to match it.

What you should do: Measure your actual PD. Using averages is gambling with your visual comfort.

Myth #2: "You Can Adapt to Wrong PD"

The myth: Even if your PD is off, your eyes will adjust after a few weeks.

The reality: The visual system can compensate for small errors through vergence adjustment and neural adaptation. But this compensation has limits.

Research shows most adults can adapt to PD errors under 2mm without lasting symptoms. Errors of 3-4mm cause persistent problems for many wearers. Errors above 4-5mm often cannot be adapted to regardless of time.

The "just give it time" advice sometimes works for minor errors and is terrible advice for significant ones.

What you should do: If symptoms persist beyond 2 weeks, don't assume you need more time. Have the glasses checked for PD accuracy.

Myth #3: "PD Is Part of Your Prescription"

The myth: PD should be included on your eyeglass prescription, and if it's not, something is wrong.

The reality: Legally, in most U.S. states, PD is not considered part of the prescription. The FTC's Eyeglass Rule requires release of prescriptions but doesn't define PD as a prescription component.

Historically, PD was considered a "fitting measurement" taken by opticians, not a "diagnostic finding" from doctors. This distinction is somewhat arbitrary but remains embedded in regulations.

What you should do: Ask for your PD separately. Don't assume its absence from your prescription means anything was done wrong—it's standard practice, even if frustrating.

Myth #4: "PD Changes Throughout Your Life"

The myth: You need to remeasure PD every time you get new glasses because it changes.

The reality: PD changes significantly only during childhood and adolescence as the skull grows. Once skeletal maturity is reached (typically by age 18-20), PD remains essentially stable for the rest of your life.

Studies of adult PD over time show changes of less than 1mm across decades. Any larger apparent "change" is more likely measurement variation than actual anatomical change.

What you should do: If you have an accurate adult PD measurement, it remains valid indefinitely. Remeasurement makes sense to verify accuracy but isn't needed to track biological changes.

Myth #5: "Online PD Tools Are Unreliable"

The myth: You can't trust app-based or web-based PD measurements; only professional measurement is accurate.

The reality: This is half-true. Poor online tools are indeed unreliable—but the best online tools achieve accuracy comparable to manual professional methods.

The key differences:

Tools requiring reference objects (credit cards) are more accurate than selfie-only tools
Tools with face alignment enforcement are more accurate than those accepting any photo
User technique matters enormously regardless of tool quality

Professional digital pupillometers are still the gold standard, but quality self-measurement tools are adequate for most purposes.

What you should do: Use reputable online tools that require reference objects and enforce alignment. Verify with multiple measurements. For high prescriptions or progressives, consider professional verification.

Myth #6: "Binocular PD Is All You Need"

The myth: The single PD number (like 63mm) is sufficient for any glasses.

The reality: For basic single vision lenses with moderate prescriptions, binocular PD usually works. For progressive lenses, high prescriptions, or anyone with significant facial asymmetry, monocular PD (separate measurements for each eye) is important.

Approximately 85-90% of people have some degree of facial asymmetry affecting eye position. Using binocular PD assumes symmetry that may not exist.

What you should do: Get monocular PD when possible—it's more complete information. Use it for progressives and high prescriptions; binocular is acceptable for basic single vision lenses.

Myth #7: "Reading Glasses Don't Need Accurate PD"

The myth: Since reading glasses are just for close-up work, PD doesn't matter much.

The reality: Reading glasses absolutely need appropriate PD—but they need near PD, which is 3-4mm less than distance PD.

When you look at close objects, your eyes converge inward. This convergence physically reduces your effective interpupillary distance. Reading glasses should be made to this reduced "near PD," not your distance PD.

Using distance PD for reading glasses means the optical centers are positioned too far apart for how your eyes are actually pointing when reading.

What you should do: When ordering reading glasses, specify near PD (distance PD minus 3-4mm), or have near PD measured specifically.

Myth #8: "Kids Can Use Adult PD Measurements"

The myth: If your child's face looks adult-sized, they can use adult PD ranges.

The reality: Children's PD is consistently smaller than adults' and follows developmental patterns:

Age 5: approximately 50mm
Age 10: approximately 55mm
Age 15: approximately 60mm

Even a teenager who looks adult-sized may have PD 4-6mm smaller than adult average. Using adult assumptions results in significantly decentered lenses.

What you should do: Measure children's PD specifically. Don't estimate from adult ranges or from the child's apparent face size.

Myth #9: "PD Errors Only Affect Clarity"

The myth: Wrong PD just makes things a little blurry; it's not a big deal.

The reality: PD errors primarily cause prismatic effect, not blur. The symptoms are:

Headaches (especially behind the eyes)
Eye strain and fatigue
Difficulty with sustained focus
Potential depth perception issues
Double vision in severe cases

Clarity might actually seem fine because the prescription power is correct—it's the positioning that's wrong. People often don't connect their symptoms to their glasses because they can see clearly.

What you should do: If you have headaches or eye strain with new glasses but vision seems clear, consider PD as a possible cause.

Myth #10: "PD Measurement Is Complicated and Requires Expensive Equipment"

The myth: Measuring PD accurately requires professional equipment costing thousands of dollars.

The reality: Digital pupillometers are convenient and accurate, but they're not the only option. Opticians have measured PD for decades using:

Simple millimeter rulers
Pupil distance rulers
Corneal reflex techniques

These methods, when performed correctly, achieve accuracy within 1mm—adequate for most purposes.

Self-measurement with a ruler, mirror, and proper technique can also achieve reasonable accuracy. It's not complicated; it just requires attention to detail.

What you should do: Don't avoid measurement because you lack fancy equipment. A millimeter ruler and good technique work fine for most people.

Bonus Myth: "If My Old Glasses Worked, Just Copy That PD"

The myth: Whatever PD was used for your previous glasses must be correct since they worked.

The reality: Your previous glasses might have used incorrect PD that you adapted to. Or they might have used correct PD that should indeed be replicated.

Simply assuming previous PD was correct perpetuates any existing errors. If you've always had mild headaches with glasses but assumed that was normal, you might have been wearing wrong PD for years.

What you should do: Verify your actual PD independently. Compare to previous glasses' optical centers. If they match, great. If they don't, consider which is actually correct.

The Truth About PD

Pupillary distance is simply a measurement of your face. It's not mystical, not complicated, and not something that should be hoarded by professionals or misunderstood by consumers.

Good information:

Your PD is a fixed number in adulthood
It should be measured, not estimated
Monocular PD is more complete than binocular
Accuracy matters more for strong prescriptions and progressives
Self-measurement can be adequate with proper technique

Misinformation:

Averages work for everyone
Adaptation fixes everything
Professional measurement is the only way
It doesn't matter much anyway

Armed with accurate information, you can navigate PD measurement confidently and end up with glasses that actually work for your face.

Ready to get your real PD? Our free online tool measures both binocular and monocular PD in seconds—no myths, just numbers.