Pupillary Distance (PD) is the measurement of the distance between the centers of your pupils, typically expressed in millimeters. This measurement is fundamental for creating eyeglasses, as it ensures that the optical center of each lens aligns precisely with your pupils. When ordering new eyeglasses, especially online, individuals often encounter situations where their PD measurement includes a decimal, leading to uncertainty about whether to round up or down.
The Rounding Dilemma
When your pupillary distance measurement is not a whole number, aiming for the most exact measurement is best. If rounding is unavoidable, rounding down to the nearest half-millimeter or whole number is preferred. For instance, if your measured PD is 63.5 mm, using 63.5 mm is ideal. If only whole numbers are accepted, 63 mm might be a safer choice than 64 mm.
This preference for rounding down relates to how light passes through corrective lenses. Each prescription lens has an optical center, a specific point where light travels without bending or distortion. For optimal vision, this optical center must align directly with the center of your pupil. Even minor discrepancies can cause light to refract incorrectly, leading to visual discomfort. Rounding down slightly rather than up helps ensure that the optical centers are positioned in a way that minimizes potential prismatic effects, where light is displaced, forcing your eyes to work harder to converge.
Importance of Accurate PD
An accurate pupillary distance is important for comfortable and clear vision through eyeglasses. If the optical center of your lenses does not align with your pupils, your eyes are forced to compensate. This can lead to eye strain, headaches, and blurred or distorted vision.
An incorrect PD can lead to a “prism effect.” This can cause your eyes to work harder to fuse images, potentially resulting in double vision, fatigue, and difficulty with depth perception. The severity of these symptoms increases with higher prescription strengths, making precise PD measurement even more important for individuals with stronger vision correction needs.
Measuring Your PD
Obtaining an accurate PD measurement is straightforward, with several methods available. The most accurate way is a professional measurement by an optician, who uses specialized tools like a pupillometer. An optician can provide this measurement during an eye exam or glasses fitting.
For home measurement, a common method uses a millimeter ruler and a mirror. Stand about 8 inches from a mirror, hold the ruler horizontally against your brow, and align the zero mark with the center of one pupil. While keeping the ruler steady, note the measurement that aligns with the center of your other pupil. Many online eyewear retailers also offer digital tools or apps that use a webcam or smartphone camera to measure PD, often requiring a standard-sized card for scale. Take multiple measurements to ensure consistency.
There are two main types of PD: single (or binocular) PD, the total distance between both pupils, and dual (or monocular) PD, which measures the distance from each pupil to the center of the nose. Single PD is sufficient for most distance vision glasses, but dual PD offers greater precision, especially for progressive lenses or facial asymmetry. Additionally, distance PD (for far vision) differs from near PD (for reading), as near PD is 2-4 mm smaller because eyes naturally converge when focusing up close. Most online orders request the distance PD.