Should I Use Near or Far PD for Progressive Lenses? Understanding Pupillary Distance for Optimal Vision
The best pupillary distance (PD) for progressive lenses depends on the specific measurement the lens manufacturer uses and the type of equipment used to take the measurements. However, generally, you should use a near PD when ordering progressive lenses. This is because progressive lenses are designed with the reading area being closer to the nose, thus requiring a more accurate near PD measurement for optimal performance, especially at close distances.
Understanding Pupillary Distance (PD)
Pupillary Distance, or PD, is the measurement of the distance between the centers of your pupils, usually expressed in millimeters. It’s a crucial measurement for correctly positioning the lenses in your eyeglasses so that you look through the optimal part of the lens for clear vision. When making eyeglasses with progressive lenses, proper PD is critical for comfortable vision.
Far PD vs. Near PD: The Key Difference
- Far PD (Distance PD): This is the measurement taken when you’re focusing on a distant object, essentially at infinity. It’s the standard PD measurement often provided on your eyeglass prescription.
- Near PD (Reading PD): This is the measurement taken when your eyes converge to focus on something close, like a book or a screen. Because your eyes move slightly inward when focusing up close, the near PD is always smaller than the far PD.
The difference between these measurements is crucial when making progressives because the near vision area of these lenses is placed lower and closer to the nose.
Why Near PD is Generally Preferred for Progressive Lenses
Progressive lenses are designed to provide clear vision at all distances – far, intermediate, and near – without the need for visible lines on the lens. The progressive corridor, a narrow channel of increasing power, connects the distance and near vision zones. Because the near vision zone is designed for focusing on close objects, a near PD measurement is typically more accurate for its placement, minimizing distortion and optimizing reading comfort.
How Near PD is Measured
There are several methods for measuring near PD:
- Manual Measurement: This involves using a PD ruler while the patient focuses on a near target, usually a letter or a point on the ruler itself.
- Digital PD Meter: These devices use sophisticated optics and software to automatically measure the near PD with greater precision. They often compensate for head tilt and parallax errors.
- Smartphone Apps (Use with Caution): While some apps claim to measure PD, their accuracy can be questionable. These should be used cautiously and verified by an eye care professional.
Common Mistakes and How to Avoid Them
- Using Only Far PD for Progressive Lenses: This can lead to eye strain, headaches, and blurry vision, particularly when reading.
- Inaccurate Measurements: Poor technique or using unreliable measuring tools can result in incorrect PD readings.
- Assuming Symmetry: While many people have symmetrical PD, some individuals have slight differences between their left and right eye measurements. A total PD reading doesn’t account for these nuances.
Factors Influencing the Choice of PD
While near PD is generally preferred, several factors influence the specific PD used for fitting progressive lenses:
- Lens Design: Some lens designs, particularly those with wider reading areas, may be more forgiving of PD inaccuracies.
- Frame Selection: The size and shape of the frame can affect the optimal PD placement.
- Doctor’s Recommendation: Always follow your eye doctor’s specific instructions regarding PD measurements. They know your individual needs best.
Benefits of Accurate PD Measurement
Accurate PD measurement is crucial for several reasons:
- Optimal Vision Clarity: Ensuring the lenses are positioned correctly in front of your eyes allows you to see clearly at all distances.
- Reduced Eye Strain: Correct PD minimizes eye strain and fatigue, especially during prolonged reading or computer work.
- Improved Comfort: Properly fitted progressive lenses are more comfortable to wear and less likely to cause headaches or dizziness.
- Faster Adaptation: Accurate PD helps you adapt to progressive lenses more quickly and easily.
Table Comparing Far and Near PD
| Feature | Far PD (Distance PD) | Near PD (Reading PD) |
|---|---|---|
| Focus Distance | Infinity | Close (e.g., 40cm) |
| Measurement | Distance between pupils when looking far away | Distance between pupils when reading |
| Typical Value | Larger | Smaller |
| Progressive Lens Use | Less accurate for reading area | More accurate for reading area |
Frequently Asked Questions (FAQs)
What happens if my PD is incorrect?
Incorrect PD can lead to a range of symptoms, including blurry vision, headaches, eye strain, dizziness, and difficulty adapting to new glasses. In severe cases, it can even cause double vision.
Can I measure my own PD at home?
While it’s possible to attempt measuring your own PD at home using a ruler and a mirror, the accuracy is often questionable. It’s always best to have your PD measured by a trained professional using calibrated equipment.
Does my PD change over time?
In adults, PD typically remains stable. However, significant changes in PD can be a sign of an underlying medical condition, and should be evaluated by an eye doctor.
Is PD the same for all types of glasses?
No. While distance glasses primarily rely on the far PD, reading glasses benefit from the near PD. Progressive lenses heavily rely on near PD due to the inclusion of reading functionality.
What if I have an asymmetric PD?
An asymmetric PD simply means the distance from your nose to each pupil is slightly different. When ordering glasses, it’s crucial to provide both the total PD and the individual measurements for each eye (e.g., Right eye PD: 32mm, Left eye PD: 30mm).
Why does my eye doctor sometimes not provide my PD?
Some eye doctors don’t routinely provide PD because they want you to purchase your glasses from their practice. However, you have the right to request your PD measurement. They are legally obligated to provide it in most jurisdictions.
Can progressive lenses correct astigmatism?
Yes, progressive lenses can correct astigmatism in addition to presbyopia (age-related farsightedness). The prescription on your lenses will specify the cylinder (CYL) and axis values needed to correct your astigmatism. Accurate PD is still necessary for proper lens function.
Are digital progressive lenses better than traditional ones?
Digital progressive lenses, also known as free-form lenses, are manufactured using computer-aided design and manufacturing (CAD/CAM) technology. This allows for greater customization and precision, resulting in wider fields of view and reduced distortion compared to traditional progressive lenses.
How long does it take to adjust to progressive lenses?
Adaptation time varies from person to person. Some people adjust within a few days, while others may take a few weeks. Wearing your new glasses consistently is key to adapting quickly.
Can I wear progressive lenses for all activities?
Progressive lenses are generally suitable for most activities. However, some people find them challenging for certain activities like playing golf or climbing stairs, due to peripheral distortion. Specific lens designs can minimize this effect. Consult with your eye doctor for the best lens options for your needs.
How often should I get my eyes examined?
It’s generally recommended to have your eyes examined every one to two years, depending on your age, health, and family history.
Is there a difference between PD and segment height?
Yes. Pupillary distance (PD) is the horizontal distance between your pupils. Segment height is the vertical measurement from the bottom of the lens to the fitting point (usually the center of your pupil) and is essential for placing the near vision portion of the lens correctly.