Eyeglasses feature a standardized system of measurements stamped directly onto the frame, which are fundamental for ensuring proper fit, comfort, and optical performance. The frame size specifications include the lens width, the temple arm length, and the Distance Between Lenses, or DBL. Understanding this numerical value is necessary for achieving a comfortable fit and optimal vision correction.
Defining DBL and Where to Find It
DBL stands for Distance Between Lenses and represents the measurement of the frame’s bridge size. It is the shortest distance, measured in millimeters, between the inner edges of the two lens openings where the frame rests on the nose. This measurement is a physical attribute of the frame, describing the width of the gap that spans the nasal area.
To find this measurement, look at the inside surface of the frame, typically on the temple arm or the bridge. The full size information is usually printed as a sequence of three numbers separated by a dash or a small square symbol (e.g., 52-18-140). The middle number (18 in this example) is the DBL measurement.
The first number represents the width of a single lens, and the third number indicates the length of the temple arm. The DBL measurement is a component of the international “boxing system,” which provides a standard for frame dimensions used across the optical industry. This standardization allows for consistent sizing across various frame styles and manufacturers.
How DBL Impacts Frame Fit
The DBL measurement directly influences how the glasses sit on the nose bridge and affects the distribution of the frame’s weight. If the DBL is too large, the frame will rest too low on the nose. This low positioning can cause the glasses to slide down and may misalign the optical center of the lenses relative to the pupil.
Conversely, a DBL that is too small will cause the frame to pinch the nose, sitting too high or too close to the face. This poor fit can result in discomfort, leave pressure marks on the skin, and compromise stability. Selecting the correct DBL is important for maintaining comfort, ensuring frame stability, and positioning the lenses at the proper height for optimal vision. The bridge width must align with the contours of the nose to prevent excessive movement and allow the weight of the frame to be distributed evenly.
DBL vs. Pupillary Distance
It is important to differentiate DBL from Pupillary Distance (PD), as they serve distinct purposes in eyewear. DBL is a fixed measurement of the physical frame, describing the width of the bridge component. This number is used solely to determine the fit and comfort of the frame on the wearer’s face and nose.
Pupillary Distance, however, is an anatomical measurement, representing the distance between the centers of the two pupils in the human eye. PD is a patient-specific measurement, expressed in millimeters, recorded during an eye examination. This number is used by the optical lab to ensure that the optical center of the lenses is ground precisely in front of the wearer’s pupils.
While both DBL and PD relate to the spacing between the eyes, they are not interchangeable. The DBL dictates the physical size of the frame’s bridge, which is necessary for frame selection. The PD is a patient’s personal biometric data, which is essential for the accurate manufacturing and centering of the prescription lenses. An incorrect PD will lead to vision problems, even if the DBL provides a comfortable fit.