What does the designation DBL indicate when used in connection with a frame?

The designation DBL stands for "distance between lenses" or more specifically, it indicates the least distance between the two lenses in a frame. This measurement is essential for proper fitting of eyewear because it ensures that the optical centers of the lenses align with the wearer's pupils. Proper alignment of optical centers is crucial for achieving optimal visual acuity and comfort.

The measurement of DBL is particularly significant when considering various frame styles and shapes, as it affects how the lenses are positioned in relation to the wearer's eyes. A correct DBL measurement contributes to aesthetic appearance as well as functional performance of the eyewear.

In contrast, other measurements such as the A (horizontal) measurement and the B (vertical) measurement pertain to the dimensions of the lenses themselves, not the spacing between them. It’s important to distinguish between these measurements, as each serves a different purpose in the fitting and performance of eyewear.