Pupillary distance (PD) is a measurement taken when ordering prescription eyeglasses. PD directly influences the clarity and comfort of vision correction. This measurement, given in millimeters, determines how successfully a pair of lenses will correct a person’s eyesight. Getting it wrong can lead to uncomfortable visual issues.
What Exactly Is Pupillary Distance?
Pupillary distance is the measured span between the centers of the two pupils, typically given in millimeters. This measurement is required by an optician to properly manufacture prescription lenses. The average adult PD ranges from about 50 to 75 millimeters.
PD is recorded in two primary ways: binocular and monocular. Binocular PD is a single number representing the total distance between the centers of both pupils. Monocular PD, also called dual PD, is a pair of numbers that measure the distance from the center of the nose bridge to the center of each pupil separately.
Monocular PD is often preferred because few people have perfectly symmetrical faces, meaning the eyes may not be equidistant from the nose bridge. Using two separate numbers ensures the lens centers are positioned precisely for each eye, accommodating facial asymmetry. PD is also measured for both far and near vision. Near PD, used for reading or close work, is typically 3 to 4 millimeters shorter than far PD because the eyes naturally converge when focusing on nearby objects.
The Critical Role of PD in Prescription Eyewear
Pupillary distance determines the optical center of the lens, which is the exact point through which light passes without distortion. For the clearest vision, this optical center must align perfectly with the center of the wearer’s pupil. The PD measurement provides the precise horizontal location for this alignment.
When the optical center is manufactured incorrectly and does not line up with the pupil, it induces an unwanted effect known as prismatic power. While a lens works by bending light, looking through any part away from the optical center bends the light in an unintended way. This forces the eye muscles to strain constantly to realign the image and fuse the two separated images into one clear picture.
This misalignment and resulting prismatic effect compromise vision comfort, even if the prescription power is correct. The effect is more pronounced for individuals with higher prescription strengths, where a small error in PD can cause major visual distortion. Accurate PD measurement is also important when fitting complex lenses, such as progressive or multifocal lenses, which require specific horizontal and vertical placement of optical zones relative to the eye.
How Pupillary Distance is Measured
PD measurement is a straightforward yet precise process that can be accomplished through various methods. Eye care professionals often use specialized instruments like a pupillometer, which is a handheld device that rests near the face and uses a corneal reflex to quickly and accurately determine the distance between the pupils. This tool often provides a superior level of accuracy, especially when measuring monocular PD.
A simpler, traditional method involves using a millimeter-scale ruler, often called a PD stick, which an optometrist or optician uses while the patient focuses on a distant object. While effective, this manual method relies more heavily on the skill of the person taking the measurement and the cooperation of the patient. For those purchasing glasses online, measuring PD at home can be done with a ruler and a mirror or with the help of a friend.
Home measurements can be useful for basic, low-power prescriptions, but they are subject to a higher degree of error. For stronger prescriptions, complex lenses, or when ordering glasses that will be worn constantly, the superior accuracy provided by a professional measurement remains the gold standard. The optical lab uses the recorded PD number to ensure that when the lens is cut and placed into the frame, the optical center sits precisely where the pupil will be looking through it.
Symptoms of Incorrect PD Alignment
When the PD used to manufacture the glasses is incorrect, the wearer experiences physical and visual symptoms due to constant eye strain from the unwanted prismatic effect. A common complaint is persistent headaches, often concentrated around the temples or eyes, as the eye muscles are overworked compensating for the misalignment. This muscle fatigue leads to generalized eye strain and discomfort.
Visual symptoms include blurred or distorted vision, even if the prescribed power is correct. Some people experience difficulty with depth perception, making it challenging to accurately judge distances, which can be concerning for activities like driving. In more severe cases of misalignment, a person may experience double vision, or diplopia, as the brain struggles to fuse the two disparate images it is receiving. It is not uncommon for people to mistakenly believe their prescription power is wrong when the issue is entirely due to an incorrectly set pupillary distance.