The small numbers engraved on your spectacle frames are not random product codes; they form the universal sizing system for eyewear. These figures, commonly presented in a sequence like 46-22-140, provide the precise dimensional measurements of the frame in millimeters (mm). Understanding this standardized system is fundamental to ensuring your glasses sit correctly on your face for optimal comfort and clear vision. The dimensions are calculated to guarantee the frame holds the lens in the exact position required by your prescription.
Decoding the Lens Width and Bridge Size
The first two numbers in the sequence, such as “46” and “22,” define the primary horizontal fit of the spectacles.
Lens Width (Eye Size)
The first number is the lens width, or eye size, which measures the horizontal diameter of the lens at its widest point. This measurement typically falls between 40 mm and 60 mm and determines the overall scale of the glasses. Frames that are too wide or too narrow may cause the face to look disproportionate.
Bridge Size
The second number, separated by a square or a dash, represents the bridge size. This is the distance between the two lenses where the frame rests on the nose, usually ranging from 14 mm to 24 mm. This measurement is crucial for preventing the frame from sitting too high or too low. A bridge size that is too large causes the glasses to slip down, while one that is too small can pinch the nose, leading to discomfort.
Identifying Temple Length and Vertical Measurement
Immediately following the lens width and bridge size is the third number, which indicates the temple length, often ranging from 120 mm to 150 mm. Temple length measures the arm of the frame from the hinge to the tip that curves behind the ear. This length keeps the glasses securely positioned on the head without causing pressure points near the temples or behind the ears.
Vertical Measurement (Lens Height)
While not always engraved on the frame, the vertical measurement, known as the “B” measurement or lens height, is a standard dimension used by opticians. This measures the vertical height of the lens at its deepest point. For wearers of multifocal lenses, such as bifocals or progressive lenses, a sufficient lens height is necessary to accommodate the different vision zones for distance, intermediate, and near viewing.
How Proper Sizing Impacts Vision and Comfort
Selecting the correct frame size directly impacts visual performance and physical comfort. An improperly sized lens width can lead to peripheral vision obstruction, especially in individuals with high prescriptions where lens edges might cause visual distortions.
The correct bridge size ensures the frame remains stable and prevents the constant need to push the glasses up, which can cause chronic skin irritation or headaches from undue pressure. An incorrect temple length affects the stability of the glasses, causing them to slide down the nose or press uncomfortably behind the ears.
The most significant optical consequence of a poor fit is the misalignment of the lens’s optical center with the wearer’s pupil. Prescription lenses are ground to provide the sharpest focus through a specific point. If the frame’s dimensions are wrong, this misalignment introduces unwanted prismatic effects that can cause eye strain and blurred vision.