Why Wrong PD Causes Headaches and Eye Strain: The Science Explained

PD and Eye Strain Headaches

Last updated: January 2025 | Reading time: 10 minutes

You got new glasses. The prescription is exactly what the doctor ordered. But after a few hours of wear, you've got a pounding headache behind your eyes. By evening, your eyes feel like they've run a marathon.

The usual suspects—screen time, stress, dehydration—don't explain it. What does? Possibly the measurement you've never thought much about: your pupillary distance.

Here's how wrong PD creates real physical symptoms, and what you can do about it.

The Mechanism: Unwanted Prism

To understand why PD errors cause discomfort, you need to understand prismatic effect.

Every prescription lens bends light in a calculated way. At the optical center—the point designed to align with your pupil—light bends exactly as intended. Your prescription is correct there.

But as you move away from the optical center, something else happens. The lens starts acting like a prism, bending light at an angle. This is called induced prism, and it's an unavoidable property of powered lenses.

When your PD is wrong, the optical centers don't align with your pupils. You're permanently looking through off-center zones where prism exists. Every moment you wear the glasses, light is being bent in ways your prescription never intended.

What Prism Does to Your Vision

Prism makes objects appear displaced from their actual position. Look through a prism (or a glasses lens off-center) at a straight line, and the line appears shifted.

For small amounts of prism, your eyes and brain compensate automatically. You don't consciously see the displacement because your visual system corrects for it.

But this correction has a cost.

Vergence Demand

Your eyes maintain single vision through vergence—the ability to point both eyes at the same target. When prism displaces the images differently for each eye, your vergence system must work harder to fuse them into one.

With correct PD, your vergence system operates in its comfort zone. With incorrect PD, it's constantly pushed beyond neutral.

Accommodation-Vergence Mismatch

Your focusing system (accommodation) and your eye-alignment system (vergence) are neurologically linked. When vergence is stressed, accommodation often responds in kind.

The result: difficulty maintaining clear focus, even though your prescription is technically correct. Your eyes struggle to find the clarity they should have.

Extraocular Muscle Fatigue

Six tiny muscles control each eye's position. When PD error forces constant vergence correction, these muscles work overtime.

Muscles that work without adequate rest fatigue. Fatigued eye muscles feel exactly like fatigued muscles anywhere else in your body: tired, sore, and achy.

The Symptoms: What PD-Induced Strain Feels Like

The clinical presentation of PD-related strain is well-documented:

Headaches

The classic PD headache is described as:

Located around or behind the eyes
Often extending to the temples
Bilateral (both sides)
Building throughout the day
Worse with extended visual tasks
Better when glasses are removed

These headaches differ from migraines (usually one-sided, with aura and light sensitivity) and tension headaches (band-like pressure around the head).

Eye Fatigue

People describe their eyes as:

"Tired" or "heavy"
Wanting to close
Sore or aching
"Strained" without specific pain

The fatigue often appears after sustained focus—reading, computer work, driving—when vergence demand is highest.

Visual Discomfort

Beyond fatigue, wrong PD can cause:

Difficulty concentrating on text
Words seeming to "swim" or "jump"
Vague sense that vision isn't sharp
Needing to squint even with new glasses
Reduced reading endurance

Associated Symptoms

Secondary symptoms that may accompany PD-induced strain:

Neck and shoulder tension (from unconscious head positioning)
General fatigue (the visual system drains mental energy)
Irritability (chronic discomfort affects mood)
Decreased work productivity

Who's Most Susceptible?

Not everyone experiences symptoms from the same degree of PD error. Susceptibility varies:

Higher Prescriptions

The math is straightforward: stronger lenses create more prism per millimeter of decentration.

A 2mm PD error creates:

0.2 prism diopters with a ±1.00 prescription
0.6 prism diopters with a ±3.00 prescription
1.0 prism diopters with a ±5.00 prescription
1.6 prism diopters with a ±8.00 prescription

Someone with strong prescription experiences four to eight times the prismatic effect from the same PD error compared to someone with a weak prescription.

Binocular Vision Vulnerabilities

Some people have naturally fragile binocular function:

Convergence insufficiency (eyes struggle to turn inward together)
Exophoria (tendency for eyes to drift outward)
Intermittent strabismus (occasional eye misalignment)

These conditions mean less reserve capacity for handling additional stress. PD errors that others tolerate may be debilitating.

Close Work Demands

The more time you spend on sustained near work, the more vergence demand accumulates. Office workers, students, readers, and crafters are higher risk for PD-related symptoms.

Age-Related Factors

Presbyopia (age-related loss of focusing ability) often coincides with reduced tolerance for optical imperfections. People who "never had problems with glasses" sometimes become symptomatic in their 40s and 50s as their visual system becomes less flexible.

Quantifying the Effect: Research Findings

Several studies have examined the relationship between PD error and symptoms:

A 2019 study in Ophthalmic and Physiological Optics surveyed 847 spectacle wearers about discomfort symptoms:

23% of those reporting chronic eye fatigue had PD errors exceeding 3mm
Symptoms correlated with error magnitude (larger errors, more symptoms)
Symptoms correlated with prescription strength

An earlier study found that most adults tolerate 2mm PD error without significant symptoms, but 4mm error produced noticeable complaints in over 60% of subjects.

Clinical guidelines generally consider:

0-2mm error: Usually asymptomatic
2-4mm error: Symptoms possible, especially with stronger prescriptions
4mm+ error: Symptoms likely for most wearers

Self-Assessment: Is Your PD Wrong?

If you're experiencing symptoms with new glasses, consider these PD indicators:

Timing Pattern

PD-related symptoms typically:

Begin shortly after starting new glasses
Build throughout the day
Improve when glasses are removed
Don't improve with continued wear (beyond normal adaptation)

If your symptoms started with new glasses and follow this pattern, PD is a plausible cause.

The Remove-and-Compare Test

Take off your glasses for 10-15 minutes. If headache and eye strain significantly improve, your glasses are causing the problem (though not necessarily via PD—it could be prescription error too).

The One-Eye Test

Cover one eye and look through your glasses with the other. Does vision seem clearer or more comfortable? Switch eyes. If one eye seems significantly more comfortable than binocular viewing, binocular alignment (potentially PD-related) may be the issue.

Measure and Compare

Measure your actual PD using reliable methods. Compare to the PD marked on your glasses (many frames have this inside the temple) or have an optician measure the glasses' optical center distance.

A discrepancy of 3mm+ combined with symptoms is strong evidence of PD-related problems.

Solutions and Next Steps

If you suspect PD-related strain:

Verify the Error

Before demanding new glasses, confirm there's actually a problem:

Measure your PD (or have it professionally measured)
Have your glasses' optical center distance measured
Compare the numbers

A concrete discrepancy gives you evidence to discuss with whoever made your glasses.

Request Remake

If PD error is confirmed, most optical providers will remake lenses at no charge. This is a manufacturing error on their part (they cut lenses to the wrong specification).

Document your symptoms, your measured PD, and the glasses' measured optical centers. Present this information when requesting correction.

Consider Adaptation

For errors under 2mm with mild symptoms, some people successfully adapt over 1-2 weeks. Your visual system can partially compensate for small errors.

However, don't force adaptation to significant errors. Chronic strain isn't good for your eyes or overall wellbeing.

Prevent Future Problems

Going forward:

Always verify your PD before ordering glasses
Keep PD records and ensure new glasses match
Request verification of optical centers on delivered glasses

When It's Not PD

PD isn't always the culprit. Other causes of new-glasses discomfort:

Prescription error: The underlying prescription is wrong Lens type change: Switching to progressives, different index materials, or new coatings can require adaptation Frame fit issues: Incorrect pantoscopic tilt, vertex distance, or frame adjustment Base curve changes: Different lens curvature from previous glasses

A systematic approach considers all possibilities. But given how often PD is overlooked, it deserves attention in any troubleshooting process.

Prevention: Getting PD Right First

The best treatment for PD-induced strain is prevention:

Know your PD before ordering glasses
Provide accurate measurements to whoever makes your lenses
Verify finished glasses against your specifications
Address discrepancies immediately rather than hoping to adapt

Your visual comfort for the next 1-2 years depends on measurements made in minutes. Investing attention in those minutes pays dividends in symptom-free wear.

Worried about your PD? Measure it free in 30 seconds with our online tool, then compare to your current glasses.