Eyeglass Lenses – Aspheric

The aspheric lens design is an important tool in helping to provide better vision to all patients.  Aspheric lenses are thinner, reduce lens weight and provide better off axis performance allowing increased visual acuity through a wider portion of the lens.

An aspheric lens is simply defined as a lens varying slightly from a perfect, spherical shape.  Why is that important to an optician when designing a lens?  When designing a lens there are a few aberrations that an optician has to deal with to provide a lens that improves the sight of a patient.  These aberrations include spherical aberration, coma, radial astigmatism, curvature of field, and distortion.  These five aberrations collectively are known as the Seidel Aberrations described by Ludwig Von Seidel in the 1850’s.  The aspheric lens is designed to correct a spherical aberration.  Here is an example:

This diagram shows a lens in front of the eye.  Look closely and you will notice that when the eye is looking straight ahead the ray has a shorter distance to focus than if the eye was rotated to look through the periphery of the lens.  This change in vertex distance, or distance between the lens and eye, has to be accounted for, otherwise objects viewed through the periphery will be out of focus.  Aspheric lenses use the change in curvature to accommodate for this change in distance.  This allows the patient to have much improved sight through the periphery of the lens.

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