Three parallel beams meeting a lens: the center beam passes straight through, the outer beams bend toward it.

Measuring and fitting

Optical center

Optical glossaryMeasuring and fitting

The point of a lens where light passes without deviation. Should sit directly in front of your pupil.

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

Light passing through a lens's optical center goes straight through; everywhere else it is bent slightly, as by a prism, and more so the further from the center and the stronger the lens. In a single vision lens the center should sit right in front of the pupil as you look ahead.

Placing it there is called centration, and it uses the PD, the fitting height, and the frame's own dimensions. When a frame's lens centers are further apart than your pupils, each lens is shifted inward in its opening — which is also why a frame that suits your PD produces thinner, better-balanced lenses.

Worked example

Lens power
+4.00
Looking off-center by
5 mm
Prism you receive
2.00 Δ

Related terms

PrismA deliberate offset that shifts an image's position, prescribed for eye-alignment problems. Unwanted prism from bad centration causes strain.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.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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