Measuring eyeglasses requires two precise sets of data: optical measurements to correct vision and physical dimensions to ensure a comfortable, functional fit. The process begins with an examination to determine the exact lens power required to focus light correctly onto the retina. This data is translated into a practical prescription and combined with precise measurements of the wearer’s face and chosen frame. Both sets of measurements are important for achieving clear vision without discomfort or eye strain.
Determining the Optical Prescription
The eyeglass prescription begins with a comprehensive eye examination that determines the degree of refractive error. This measurement process, known as refraction, has two phases: objective and subjective.
Objective measurements are taken first, using instruments like an auto-refractor or a retinoscope. These devices measure how light travels through the eye without requiring patient feedback. The auto-refractor calculates the preliminary power needed to focus light sharply onto the retina, giving the professional a starting point.
Subjective refinement, or subjective refraction, involves the patient’s direct feedback to fine-tune the lens powers. The patient sits behind a phoropter, a device containing numerous lenses, and looks at an eye chart. The professional presents lens choices, asking which option provides clearer vision. This systematic comparison, often in small increments of 0.25 diopters, determines the most accurate lens power for the best visual acuity.
Decoding the Prescription
The final output of the eye examination is a written prescription detailing the necessary lens powers. The letters OD (Oculus Dexter) and OS (Oculus Sinister) specify measurements for the right and left eyes, respectively.
The SPH, or Sphere, column indicates the main lens power required to correct nearsightedness or farsightedness, measured in diopters. A minus sign (-) signifies nearsightedness, while a plus sign (+) indicates farsightedness.
The CYL (Cylinder) and AXIS measurements are present only if the patient has astigmatism, a condition where the cornea is irregularly shaped. The CYL number specifies the extra lens power needed to correct this curve. The AXIS is a number between 1 and 180 degrees that dictates the exact angle at which the cylinder correction must be placed on the lens.
Measuring Pupillary Distance
Pupillary Distance (PD) is a measurement separate from the optical prescription but important for lens fabrication. This measurement is the distance, in millimeters, between the centers of the two pupils. An accurate PD ensures that the optical center of the lens—the point with the full prescribed power—aligns perfectly with the center of the wearer’s pupil.
If the PD is incorrect, the wearer may experience eye strain, blurred vision, or headaches. PD is measured in two ways: binocularly, as a single number representing the distance between both pupils, or monocularly, as two separate numbers. Monocular PD measures the distance from the nose bridge center to the center of each pupil. Monocular PD is often preferred because slight facial asymmetry makes two separate measurements more accurate for optimal lens alignment.
Sizing the Frame for Comfort and Fit
The final step involves measuring the frame itself to guarantee a comfortable and secure fit on the wearer’s face. Frame dimensions are standardized and typically stamped as a series of three numbers on the inside of the temple arm or the bridge. These measurements are always given in millimeters.
The first number is the Lens Width (eye size), which is the horizontal width of the lens opening, usually ranging from 40 to 60 mm. The second number is the Bridge Width, representing the distance between the two lenses where the frame rests on the nose, commonly between 14 and 24 mm. A proper bridge width prevents the glasses from sliding down or pinching the nose.
The third number is the Temple Length, which measures the length of the arm from the hinge to the tip that rests behind the ear, typically between 120 and 150 mm. The correct temple length ensures the frame sits securely without causing pressure on the side of the head or behind the ear. These three measurements provide a frame that supports the lenses correctly and remains comfortable.