A lens in a stand and an eye-sized glass sphere behind it, a sliver of light in the gap between them.

Measuring and fitting

Vertex distance

Optical glossaryMeasuring and fitting

The gap between the back of the lens and your eye. At strong powers, a change in this distance changes the effective prescription.

Three measurements of how a frame sits: vertex distance, the gap from lens to eye; pantoscopic tilt, how far the lens leans in at the bottom; and wrap, how far the frame curves round your face.

Illustrative · the shape of the principle, not measured data

In more depth

Vertex distance is the gap between the back surface of the lens and the front of the eye. Prescriptions are tested at one distance and worn at another, and for weak lenses the difference is irrelevant. For strong lenses it is not: moving a lens changes the power that actually reaches the eye.

Moving a minus lens closer to the eye makes it act stronger; moving a plus lens closer makes it act weaker. Beyond about four diopters a few millimeters starts to count, which is why the same prescription is converted when it becomes a contact lens, and why individualized designs ask for the real distance in your frame.

Worked example

Prescribed power
−8.00
Measured at
12 mm
Worn at
0 mm
The same correction needs
−7.30

Related terms

Pantoscopic tiltThe forward tilt of a frame relative to the face. Individualized designs account for it; standard designs assume an average.Wrap angleHow much a frame curves around the face. High-wrap sports frames need designs compensated for it.Freeform surfacingManufacturing in which a lens surface is cut point by point by a computer-guided tool from a map calculated for one order, rather than molded. On made-to-order CLIFF lenses it is the back surface — the surfacing and the design both — on every made-to-order tier. Versatile, the stock lens, is molded, not freeform.

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