A frame on a leather fitting mat, a fine thread stretched between dots at the center of each lens.

Measuring and fitting

Pupillary distance (PD)

Optical glossaryMeasuring and fitting

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.

Each lens is cut so its optical center sits in front of your pupil. When your pupils sit closer than the frame's centers, each center moves in — and the thicker part of the lens moves to its outer edge.

Illustrative · the shape of the principle, not measured data

In more depth

PD is measured between the centers of the pupils, in millimeters. Most adults fall somewhere in the mid-fifties to low seventies. It is taken looking into the distance; a separate, shorter near PD is used for reading glasses, because the eyes turn inward to read.

The figure tells the lab where to put each lens's optical center. Get it wrong and you look through the lens away from its center, which introduces prism the prescription never asked for — felt as strain, headaches or a pulling sensation rather than blur.

Related terms

Monocular PDYour PD measured separately for each eye, from the nose bridge. More accurate than a single figure, and required for individualized lenses.Optical centerThe point of a lens where light passes without deviation. Should sit directly in front of your pupil.PrismA deliberate offset that shifts an image's position, prescribed for eye-alignment problems. Unwanted prism from bad centration causes strain.Frame measurementsThe 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.

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