Accurate measurements are necessary when acquiring new eyeglasses, particularly when purchasing them through online retailers. There is a distinction between clinical measurements, which determine lens power, and physical measurements, which ensure the lenses are aligned precisely with your eyes and the frames fit comfortably. While only a licensed professional can determine the necessary power correction, several physical measurements regarding frame fit and eye alignment can be taken outside of a clinical setting. Understanding both sets of numbers is fundamental to receiving glasses that provide clear and comfortable vision.
Understanding Your Professional Eye Prescription
The starting point for any pair of corrective lenses is the professional eye prescription (Rx), obtained through a comprehensive eye examination. This document contains precise values describing the optical power required to correct your specific refractive error. These values are determined by an optometrist or ophthalmologist using specialized equipment and cannot be measured at home.
The most common abbreviations found on an Rx are SPH, CYL, and Axis. The Sphere (SPH) value indicates the main lens power needed to correct nearsightedness (minus sign) or farsightedness (plus sign). The Cylinder (CYL) and Axis values correct astigmatism, an imperfection in the eye’s curvature. The CYL specifies the corrective power needed, and the Axis (1 to 180) denotes the orientation where this power is applied.
The Add Power (ADD) is required for multifocal lenses, such as bifocals or progressives, to provide magnification for reading and close-up tasks. This measurement is always a positive value. Since these numbers determine the curvature and power ground into the lens material, they must be acquired from a professional eye care provider.
The Critical DIY Measurement: Pupillary Distance (PD)
The most significant physical measurement you can take at home is the Pupillary Distance (PD), the distance between the centers of your two pupils. This measurement ensures the optical center of each corrective lens aligns perfectly with the center of your eye for clear and comfortable vision. An improperly centered lens, even with the correct prescription, can induce prism effects that lead to eye strain, headaches, or double vision.
The PD is recorded in millimeters (mm) and can be expressed as a single PD or a dual PD. A single PD is one number representing the total distance between the pupils (typically 54 mm to 74 mm in adults). A dual PD provides two separate numbers (e.g., 32/31), representing the distance from the center of the nose bridge to the center of each pupil individually. The dual measurement is more precise because it accounts for slight facial asymmetries.
Measuring PD with a Mirror
One practical method involves a ruler and a mirror. Stand approximately arm’s length from the mirror, holding a millimeter ruler horizontally across the bridge of your nose. Close your right eye and align the ruler’s zero mark over the center of your left pupil. Without moving your head or the ruler, open your right eye, close your left eye, and read the mark that aligns with the center of your right pupil; this reading is your single PD.
Measuring PD with a Partner
A more accurate method is to have a friend or partner take the measurement. You must look straight ahead at a distant object at eye level, keeping your gaze steady, as looking near causes pupils to constrict and move inward. The partner should stand an arm’s length away and use the ruler to measure the distance between the center of one pupil and the center of the other. For a dual PD, the partner measures from the center of the nose bridge to the center of each pupil separately.
Decoding Frame Sizing and Fit Measurements
The fit of the frame is determined by three standardized measurements typically stamped on the inside of the frame arm or bridge (e.g., 50-20-140). These numbers dictate the physical dimensions of the frame in millimeters. Understanding these dimensions allows you to select a frame size that sits comfortably and correctly on your face.
The first number (50) is the Lens Width or Eye Size, the horizontal diameter of the lens measured at its widest point. The second number (20) is the Bridge Width, the distance between the two lenses. This measurement dictates how the frame rests on the bridge of your nose.
The final number (140) is the Temple Length, which measures the length of the arm from the hinge to the tip, including the portion that curves around the ear. This length determines if the arms will hook securely behind your ears. If these numbers are worn off, you can use a millimeter ruler to measure them directly. Matching these three measurements to your current comfortable frames ensures a correct fit.
Limitations of DIY Measurement and Complex Lenses
While measuring PD and frame size at home works for basic single-vision lenses, DIY methods have limitations with complex prescriptions or lens types. High prescriptions (Sphere or Cylinder values exceeding +/- 4.00 Diopters) require precise measurements. Inaccuracies in PD that are negligible for a mild prescription can cause visual disturbances when dealing with stronger corrective powers.
Progressive and Bifocal Lenses
Progressive or bifocal lenses require an additional measurement known as segment height (or fitting height). Segment height is the vertical distance from the bottom of the lens to the point where the reading portion begins. This measurement is unique to the frame and the wearer’s gaze. It must be taken while the wearer is looking straight ahead in the chosen frame. DIY attempts to determine this height are prone to error and can result in unusable or uncomfortable lenses.
Prism Correction
Prescriptions that include a Prism correction, used to correct eye alignment problems like strabismus, require professional expertise. Prism lenses shift the image seen by the eye, and the placement and orientation of this prism are sensitive. Attempting to measure the necessary parameters for these complex lenses at home can lead to distorted vision and exacerbate underlying eye alignment issues.