Vertex distance

Vertex distance is the distance between the back surface of a corrective lens, i. e. glasses (spectacles) or contact lenses, and the front of the cornea.

Increasing or decreasing the vertex distance changes the optical properties of the system, by moving the focal point forward or backward, effectively changing the power of the lens relative to the eye.
Since most refractions are performed at a vertex distance of 12–14 mm, the power of the correction may need to be modified from the initial prescription so that light reaches the patient's eye with the same effective power that it did through the phoropter or trial frame.

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Source: Wikipedia · Image: Wikimedia Commons · CC BY-SA 4.0

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