The numbers associated with a pair of glasses are precise measurements that serve two different purposes. One set of figures dictates the exact optical power needed to correct a person’s vision, ensuring light focuses properly on the retina. The second set of numbers relates to the physical dimensions of the frame, which guarantees a comfortable and proper fit on the face. Both measurements are necessary for effective prescription eyewear.
Decoding the Core Vision Correction Values
The primary optical figures on a prescription determine the power of the lens and are measured in diopters. The most fundamental value is the Sphere (SPH), which indicates the main strength of correction needed for nearsightedness or farsightedness. A minus sign (-) next to the SPH value signifies nearsightedness, while a plus sign (+) indicates farsightedness. The further the number is from zero, the stronger the required lens correction.
The prescription also includes abbreviations to specify which eye receives which correction. OD (Oculus Dexter) is the Latin term for the right eye, and OS (Oculus Sinister) refers to the left eye. Each eye often requires a slightly different corrective measurement. If both eyes require the same prescription, the abbreviation OU (Oculus Uterque) might be used to indicate both eyes.
For individuals with astigmatism (an imperfection in the curvature of the eye’s lens or cornea), two additional values are included: Cylinder (CYL) and Axis. The CYL figure specifies the extra lens power needed to correct this shape. This value is measured in diopters and is left blank if a person does not have astigmatism.
The Axis number indicates the orientation or angle at which the CYL correction must be applied to the lens. This value is given in degrees, ranging from 1 to 180. 90 degrees represents the vertical position, and 180 degrees represents the horizontal position on the eye. The Axis value is only present on a prescription if a CYL value is also present, as it describes the precise placement of the astigmatism correction.
Understanding Frame Size Specifications
Separate from the vision correction values, standardized numbers are found on the inside of the frame, typically printed on the temple arm or the bridge. These measurements describe the physical frame and ensure proper fit and comfort. The three most common figures are measured in millimeters and are usually separated by a dash or a small square, such as in the sequence 52-18-140.
The first number in this sequence is the Lens Diameter (or eye size), which measures the horizontal width of the lens opening at its widest point. This size can range from approximately 40mm for narrow frames up to over 60mm for oversized styles. The lens diameter dictates the overall width of the frame and where the wearer’s eye will center within the lens.
The second number indicates the Bridge Width, which is the distance between the two lenses where the frame rests on the nose. This measurement, often between 14mm and 24mm, is essential for a comfortable fit. A bridge that is too narrow will pinch the nose, and one that is too wide will cause the glasses to slide down. The correct bridge width ensures the weight of the glasses is distributed properly.
The third number represents the Temple Length, which is the measurement of the arm piece from the hinge to the tip that curves behind the ear. Standard temple lengths commonly fall between 135mm and 150mm. This length determines how securely the glasses sit on the head and prevents pressure on the ears.
The Essential Measurement for Lens Placement
Pupillary Distance (PD) is a separate measurement that connects the optical prescription to the physical frame and is essential for lens alignment. PD measures the exact distance in millimeters between the centers of the pupils of both eyes. This figure ensures the optical center of the prescription lens lines up precisely with the center of the pupil.
If the lens is not centered correctly over the eye, even a perfectly calculated SPH and CYL power can cause visual discomfort. An incorrect PD forces the wearer to look through a sub-optimal area of the lens, which can lead to eye strain, headaches, or double vision. The consequences of inaccurate alignment are particularly noticeable in higher prescriptions and in multifocal lenses, which require precise positioning for different viewing zones.
Sometimes, a single PD number is provided, representing the total distance between the two pupils. However, some prescriptions use a Dual PD (or monocular PD), which provides two separate numbers. These numbers measure the distance from the center of each pupil to the bridge of the nose. This dual measurement allows for a more personalized alignment, especially if a person’s pupils are not perfectly centered.