Accurate Pupillary Distance Measurement: Why It Matters

Eye care specialists strongly recommend updating your glasses every one to two years. When you visit your eye doctor to get new prescription eyewear, they will perform several tests to detect changes in your vision. Before making your glasses, expect an optician to measure your pupillary distance (PD).  

Read on to discover the importance of accurate pupillary distance measurements.

What Is Pupillary Distance?

You will not find pupillary distance on your eyeglass prescription, but it plays a significant role in the effectiveness of your glasses. It refers to the space between your pupils, the central part of the eyes that allows light to pass through. Your pupils also widen to help you focus on a nearby object and see in dark or very bright environments. 

PD is crucial to the proper alignment of your corrective lenses. It ensures that the optical center of the lenses lines up correctly with your pupils. Only trained eye care professionals can accurately measure PD since the task requires extensive knowledge and technical skills. Opticians have advanced tools that let them take correct measurements. 

Types of Pupillary Distance

Far Pupillary Distance

When looking at a faraway object, the measurement between the centers of your two pupils is called far pupillary distance. Your eye care specialist will take your PD to determine if you need new prescription glasses. They might not check it if you are only getting reading glasses. 

Near Pupillary Distance

Near pupillary distance refers to the proximity of the center of your pupils when you gaze at a nearby object. Typically, it’s around 3-4 mm shorter than your far pupillary distance. 

Single vs. Dual Pupillary Distance

Binocular Pupillary Distance

Single or binocular pupillary distance measures the distance between the centers of your pupils. It only pertains to one number or refers to both eyes, and it’s the usual measurement method opticians use. 

Monocular Pupillary Distance

Dual or monocular pupillary distance is the length from the center of one pupil to your nose bridge. Dual PD has two figures, one referring to the left eye and another to the right. It isn’t uncommon for one side to have a slight variation from the other. 

The Importance of Pupillary Distance

Ensuring Good Eyeglass Fit

Measuring pupillary distance is a part of your eye examination because it helps eye doctors provide you with the most suitable vision correction glasses for your needs. The center of your lenses must line up with the front of your pupils for comfortable wear. It’s the clearest area of your prescription, meaning aligning it with your pupil helps you see clearly and avoid eye strain. 

Opticians often use a pupillometer or digital PD meter to measure pupillary distance. Sometimes, they bring out a special ruler to measure your PD or mark a pair of eyeglasses and assess the distance between them. 

Assessing Near-Point Convergence

Near-point convergence happens when your eyes focus on an object close by. Your eye doctor might bring an object to your nose to examine your eyes’ ability to focus. As your eyes come together, your pupillary distance shortens.  

Checking Stereo Acuity

PD helps eye doctors assess your stereo acuity or depth perception. Each of your eyes sees objects differently. However, your brain combines the images from your left and right visions to create one three-dimensional image. With the help of PD, eye care providers can understand how your visual field varies between your left and right eyes. 

Consequences of Incorrect PD Measurements

Headaches

An inaccurate PD can cause the light that passes through your retina to bend incorrectly. As a result, your brain’s visual cortex has to work harder to fix the incorrect visual cue. Over time, the added load on your brain can result in headaches. Frequent headaches might discourage people from wearing their glasses, further impacting vision.

Eye Strain and Blurred Vision 

When the lenses of your eyeglasses don’t align with your pupils, the eyes must put in extra effort to focus. It can lead to eye strain, causing discomfort while wearing prescription glasses. Another issue an incorrect PD can cause is blurred or double vision. It can result in difficulties performing tasks that require your eye to focus, like reading, driving or using a computer. 

Reduced Visual Acuity

Visual acuity refers to the sharpness and clarity of your vision. Accurate PD is crucial for maintaining optimal visual acuity. Glasses with incorrect PD measurements can make it harder to see fine details, read small print or recognize faces from a distance.

Difficulty Adapting to New Glasses

Your eyes usually need a few days to adjust to your new glasses. During this period, it is normal to experience some eye strain, slightly blurred vision or dizziness. If your symptoms last more than a couple of days or a week, it is best to contact your eye care provider. They might schedule another eye exam appointment to determine if there were inaccuracies with your eyewear’s measurements.

Importance of Regular Eye Exams

Detect Vision Changes 

Your eyes go through a lot of changes as you get older. Regular comprehensive eye exams can detect the slightest changes in your vision. They help eye doctors provide patients with appropriate prescription lenses. You can see clearly and avoid eye strain with the proper eyewear.

Monitors Overall Eye Health

Besides identifying changes in your vision, regular eye exams can help monitor your overall eye health. They are essential to the early detection and treatment of many vision-threatening eye conditions. Glaucoma and macular degeneration are two eye problems that don’t often manifest symptoms until in their later stages. Catching and treating them early goes a long way in preserving your vision. Moreover, comprehensive eye exams can help diagnose other health issues, including diabetes and hypertension.