The inside of a tortoiseshell temple arm and its hinge, in close-up.

Measuring and fitting

Frame measurements

Optical glossaryMeasuring and fitting

The code inside the temple arm (e.g. 51□19-145): lens width, bridge width, and temple length in millimeters. More than frame trivia — lens width drives edge thickness at stronger prescriptions, and together with the bridge it determines how far the lenses must shift to center on your pupils.

  • 48 mm lens width: edge 3.9 mm
  • 56 mm lens width: edge 4.8 mm
The same −5.00 prescription in a 48 mm and a 56 mm lens width, in the same material. The wider lens reaches further from the center, where a minus lens is thickest.

Worked example · standard optics, computed

In more depth

The three numbers printed inside a temple arm follow the boxing system: the width of each lens opening, the bridge between them, and the length of the arm, all in millimeters. The small square between the first two means the lens was measured across a box drawn around its shape.

Lens width plus bridge gives the distance between the frame's lens centers. When that is wider than your PD, each lens has to be shifted inward to center on your pupil, which pushes the thicker part of the lens toward the outer edge — the edge people see.

Worked example

Lens width
51 mm
Bridge
19 mm
Frame centers apart
70 mm
Your PD
62 mm
Each lens shifts inward
4 mm

Related terms

Pupillary distance (PD)The distance between your pupil centers in millimeters. It positions each lens's optical center in front of each eye; errors cause strain even with a correct prescription.Optical centerThe point of a lens where light passes without deviation. Should sit directly in front of your pupil.High indexIn the CLIFF range, materials of 1.67 and above, used to keep strong prescriptions thin. Buys thinness, not clarity.

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