PD for Reading Glasses: Why Near Vision Needs Different Numbers

PD for Reading Glasses

Last updated: January 2025 | Reading time: 8 minutes

You've measured your PD. It's 64mm. You order reading glasses with 64mm PD. And they don't feel quite right—a bit of strain, the sweet spot seems narrow, reading isn't as comfortable as it should be.

What went wrong? You used your distance PD for near-vision glasses. That's like using highway directions when you need parking lot directions—close, but not right.

The Convergence Factor

When you look at something far away, your eyes point essentially parallel. Each eye looks straight ahead at the distant target.

When you look at something close—like a book at 40cm—your eyes must turn inward to both aim at the same near point. This inward turning is called convergence, and it physically reduces the distance between your pupils.

The closer the target, the more convergence required, and the smaller your effective PD becomes.

This isn't a defect or a variation—it's how human vision works. Your distance PD and your near PD are both accurate; they just measure different things.

How Much Difference?

The difference between distance PD and near PD depends on:

Your distance PD (starting point)
The working distance (how close you hold reading material)

For typical reading distance (40cm or about 16 inches), the convergence effect reduces PD by approximately 3-4mm.

Example:

Distance PD: 64mm
Reading PD at 40cm: approximately 60-61mm

For computer distance (typically 50-70cm), the reduction is smaller:

Distance PD: 64mm
Computer distance PD: approximately 62-63mm

The Formula (If You Want It)

Opticians calculate near PD using this relationship:

Near PD = Distance PD - (Distance PD × Working Distance ÷ 1000)

Where working distance is in millimeters.

For 64mm distance PD at 400mm working distance: Near PD = 64 - (64 × 400 ÷ 1000) = 64 - 2.56 = 61.44mm

This formula gives a rough approximation. Individual variation in convergence ability means your actual near PD might differ slightly.

When Near PD Matters

Dedicated Reading Glasses

If you're ordering glasses specifically for reading—whether readers from a drugstore or prescription reading glasses—near PD is what you need.

The lenses are designed for close focus. Your eyes will be converged when using them. The optical centers should align with where your pupils actually are during that converged state.

Using distance PD for reading glasses means the optical centers are positioned too far apart for your converged eyes. You're looking through slightly off-center portions of each lens, inducing unwanted prism.

Single Vision Near/Intermediate Glasses

Similarly, computer glasses or task-specific near-vision glasses should use an appropriate PD for their intended working distance.

Computer glasses might use a PD between distance and near values, depending on your typical screen distance.

Dedicated Bifocals (Non-Progressive)

Traditional lined bifocals have distinct distance and near segments. The near segment should be positioned for near PD, while the distance portion uses distance PD.

Optical labs understand this—when you order bifocals, they typically apply appropriate "inset" to the near segment automatically based on standard calculations. But if you're providing your own measurements, awareness helps.

When Distance PD Is Used

Progressive Lenses

Progressives are ordered using distance PD. The lab calculates the inset for the near zone based on the lens design and your add power. You provide distance PD; they handle the rest.

This is why progressives are more complex than single vision lenses—the corridor must smoothly transition not just in power but in horizontal positioning to accommodate natural convergence.

Distance Glasses

Obviously, glasses for distance vision (driving, general wear for nearsighted people) use distance PD.

Most "General Purpose" Orders

When ordering online and the form just asks for "PD" without specifying near or distance, they almost certainly mean distance PD. This is the industry default.

If you're ordering reading glasses from such a site, you may need to enter your calculated near PD rather than your distance PD, or contact customer service to clarify their expectation.

Practical Guidance

If You're Ordering Reading Glasses:

Start with your distance PD (measured or known)
Subtract 3-4mm for standard reading distance
Enter the calculated near PD when ordering

Example: Distance PD is 66mm. Near PD for reading = 66 - 3.5 = 62.5mm (round to 62 or 63mm)

If You're Ordering Computer Glasses:

Estimate your typical screen distance:

50cm (20 inches): subtract ~2mm from distance PD
60cm (24 inches): subtract ~1.5mm from distance PD
70cm+ (28+ inches): use distance PD or subtract 1mm

If You're Getting Progressives:

Provide distance PD. The optical lab handles near zone positioning as part of the progressive design. You don't need to calculate anything.

If You're Unsure:

Ask the seller. Clarify: "Should I provide distance PD or near PD for reading glasses?" A reputable retailer will answer clearly.

The Over-the-Counter Reading Glasses Problem

Drugstore reading glasses are made with a single, fixed PD—usually around 62-64mm, targeting the average adult.

If your near PD differs significantly from this assumption:

Too wide (your near PD is 58mm, glasses are 64mm): optical centers are too far apart
Too narrow (your near PD is 70mm, glasses are 64mm): optical centers are too close together

Either way, you're dealing with induced prism that may cause eye strain during extended reading.

This is one reason cheap reading glasses work fine for some people and cause headaches for others. It's not (only) about optical quality—it's about whether the one-size-fits-all PD happens to fit your face.

Monocular Near PD

Just as distance PD can be expressed as monocular (separate values for each eye), so can near PD.

The convergence reduction applies proportionally to each eye. If your distance monocular PD is 32/34, your near monocular PD would be approximately 30.5/32.5 (each eye converges by ~1.5mm at reading distance).

For basic reading glasses, binocular near PD is usually sufficient. For higher prescriptions or if you're sensitive to optical imperfections, monocular near PD provides more precision.

Measuring Near PD Directly

You can measure near PD directly instead of calculating from distance PD:

Method:

Hold a millimeter ruler at reading distance (where you'd hold a book)
Look at the ruler with both eyes while someone else observes
Have the observer note where your pupils align with the ruler markings

The challenge is getting both your focus point and the measurement plane aligned. It's trickier than distance PD measurement, which is why calculation from distance PD is often easier.

Photo Method:

Take a photo while looking at a close object positioned just behind the camera. Include a reference card. Your converged pupils will show in the photo, allowing direct measurement.

The Takeaway

Distance PD and near PD are both real, valid measurements of your eyes—at different focus distances. Using the wrong one creates positioning errors that can cause strain and discomfort.

For reading glasses and near-vision tasks: use near PD (distance PD minus 3-4mm) For distance glasses and progressives: use distance PD

It's a simple adjustment that makes a meaningful difference in how your reading glasses perform.

Need your near PD? Start with your distance PD from our free measurement tool, then subtract 3mm for reading distance.