Acquiring new eyeglasses requires two distinct types of measurement: the clinical determination of the precise optical power needed to correct vision and the physical measurement of the frame to ensure a proper fit. Accuracy in both steps is necessary for effective vision correction, as a small error can lead to issues like eye strain or blurred vision.
Determining the Optical Prescription
The clinical measurement of vision correction, known as refraction, is performed by an eye care professional, often using a device called a phoropter. This instrument contains a series of lenses that the professional rapidly switches in front of the patient’s eyes to determine the sharpest possible vision. The final result of this process is the optical prescription, which is measured in units called diopters.
The prescription is broken down into three main components for each eye, often listed as Sphere (SPH), Cylinder (CYL), and Axis. The Sphere value indicates the lens power needed to correct for nearsightedness or farsightedness. A negative sign signifies correction for nearsightedness (myopia), while a positive sign indicates correction for farsightedness (hyperopia).
The Cylinder and Axis values address astigmatism, a condition where the cornea or lens is not perfectly spherical, leading to blurred vision. Cylinder specifies the power needed to correct this irregular curve, and Axis defines the orientation of that correction, measured in degrees from 1 to 180. This combination of SPH, CYL, and Axis determines the precise curvature ground into the lens surface to focus light correctly onto the retina.
Measuring Pupil Distance
The Pupillary Distance (PD) is the space between the centers of the two pupils. This measurement ensures the optical center of the finished lens—the spot where the prescription power is delivered—is aligned perfectly with the center of the wearer’s pupils. If the lens center is misaligned by even a few millimeters, it can induce an unwanted prism effect, potentially causing eye strain, headaches, or double vision.
The PD is measured in millimeters and is taken by the eye care professional using a specialized ruler or a digital measuring device. For adults, the average PD is around 63 millimeters, generally ranging between 50 mm and 75 mm. In some cases, a monocular PD is measured, which provides two separate values: the distance from the center of the nose bridge to the center of each pupil. This dual measurement is preferred for higher prescriptions or frames with asymmetrical designs, allowing for greater precision in lens placement.
Understanding Frame Dimensions
The physical fit of the glasses frame is determined by a series of measurements usually printed on the inside of the temple arm, such as 50-18-140. These three numbers are measured in millimeters. The first number, called the eye size (or ‘A’ measurement), is the horizontal width of the lens opening. This number determines how wide the frame sits across the face, with adult sizes typically ranging from 40 to 60 millimeters.
The second number is the bridge width, which is the distance between the two lenses where the frame rests on the nose, often referred to as the Distance Between Lenses (DBL). This measurement is key to comfort, as a bridge that is too narrow will pinch, and one that is too wide will cause the glasses to slip down the nose. Bridge widths typically fall between 14 and 24 millimeters.
The third number indicates the temple arm length, measured from the hinge to the tip that curves behind the ear. Standard temple lengths range from 120 mm to 150 mm, and the correct length prevents the glasses from being too tight or too loose. The frame also has a vertical measurement, known as the ‘B’ measurement, which is the height of the lens opening and is important for positioning multifocal lenses.