What is Pupillary Distance?
Pupillary distance (PD) is a measurement of the distance between the centers of the pupils. It is used to determine where someone looks through the lens of their glasses.
Measuring PD accurately results in a better lens prescription and improved overall well-being. Incorrect PD causes blurred vision, double vision, headache, fatigue, and nausea.
Even a small error in PD measurement can lead to occasional mild headaches.
Unfortunately, many people experience mild symptoms but never realize an incorrect PD measurement is the cause. They assume that wearing prescription glasses is enough to correct their vision issues and never know that their glasses are poorly fitted.
What is the Average Pupillary Distance?
The average adult’s PD is between 54 and 75 mm. The average for children is 43 to 58 mm.
What is Pupillary Distance Used For?
Pupillary distance is used to design prescription eyeglasses. Aligning the optical center of the lenses of your glasses results in comfort and visual clarity.
For those with a low myopia prescription and no need for astigmatism correction, PD is less of a factor. Being within a millimeter or two is enough to alleviate problems.
However, for those with a high myopia prescription, especially if there is a need for astigmatism correction, PD measurement is critical.
The further you drift from the optical center of your lenses, the more light will hit the retina of the eye and bend it in various ways. A mistake in the measurement of PD creates blurriness.
What Determines Your Pupillary Distance?
PD measurement determines how far one’s pupils are apart. This determines the optical center of the lenses of glasses prescriptions. PD is determined by the spacing of the pupils.
Single vs. Dual Pupillary Distance: What’s the Difference?
There is single (monocular) and dual (binocular) pupillary distance. Both are important when your doctor is measuring you for eyeglasses.
Single PD is the distance between the center of the pupil and the middle of the nose bridge. Doctors measure single PD when fitting progressive lenses.
Dual PD is the distance between the center of the pupil of one eye and the center of the pupil in the other eye.
How to Measure Your Pupillary Distance
Most people have their PD measured when visiting their eye doctor and getting fitted for a new pair of prescription eyeglasses. It’s also possible to measure PD at home. You do this by:
- First, stand 8 inches from a mirror and hold a millimeter ruler against your brow bone with your eyes open
- Close your right eye and align the ruler’s 0 mm with the center of your left pupil
- Looking straight ahead into the mirror, close your left eye and open your right eye
- The mm line that lines up with the center of your right pupil is the PD measurement
There is another method for measuring PD if you already wear prescription eyeglasses. You’ll need a non-permanent marker.
- With your glasses on, stand about 12 inches from a mirror.
- Close your right eye and put a small mark on the left lens of your glasses in front of your pupil
- Holding your head still, close your left eye and open your right eye
- Looking at your reflection in the mirror, place a small mark on the right lens as you did the left, right in front of your pupil
- Remove your glasses and measure the distance between the two dots to determine your PD
You can compare the measurement you take to what you find on your prescription or what’s in your medical history.
If you believe your PD was measured incorrectly when you were fitted for eyeglasses or you don’t feel as if your glasses fit you correctly, speak to your doctor about your PD. Together, you can determine if PD is a problem and if your glasses need to be adjusted for better visual health.
How to Find Pupillary Distance on Prescription
Your pupillary distance is also found on your eyeglasses prescription. Not all prescriptions include PD. If yours does, it will be a number with a “PD” written beside it.
Prescription for contact lenses likely won’t feature a measurement for pupillary distance.
If you aren’t sure where to find your PD, ask your doctor for information.
Does Pupillary Distance Affect Eyeglass Frame Size?
Yes. Pupillary distance affects the size of your glasses' frames but not the shape. PD influences the shape and size of your lenses.
Frame measurements are sometimes overlooked when prescribing eyeglasses. Different frames affect PD because the size of the frames affects the optical center. This is especially true with larger lenses and frames.
Finding a properly fitted frame for prescription lenses is just as important as getting an accurate PD measurement. To prevent headaches and other side effects associated with your vision, it’s important to bring up any concerns you have with your doctor. They can re-measure or check the fit of your eyeglasses to determine if there is a problem.
Your eye doctor or optician will help determine the right frames for you. If you order glasses online, you’ll want to use an old pair of properly fitted glasses.
The five areas to consider when measuring your frames for an accurate fit include:
- Frame width
- Lens height
- Lens width
- Temple arms
- Bridge
Proper PD measurement provides your eyes the best opportunity to focus. An accurate measurement drastically enhances the overall experience of wearing corrective lenses. Your eye doctor or other eye care professional will measure PD as part of a routine eye exam and when they are writing a prescription for eyeglasses.
Your PD number is an important part of your prescription and ensures that you don’t suffer eye strain or experience headaches and other side effects of poorly fitted eyeglasses.
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