Confusing corrective lens industry terminology




Spheric vs. aspheric, atoric, etc.edit

Lens manufacturers claim that aspheric lenses improve vision over traditional spheric lenses. This statement could be misleading to individuals who do not know that the lenses are being implicitly compared to "a spheric flattened away from best-form for cosmetic reasons".verification needed This qualification is necessary since best-form spherics are always better than aspherics for an ophthalmic lens application. Aspherics are onlyverification needed used for corrective lenses when, in order to achieve a flatter lens for cosmetic reasons, the lens design deviates from best-form sphere; this results in degradation of the visual correction, degradation which can, in some part, be compensated for by an aspheric design. The same is true for atoric and bi-aspheric.

While it is true that aspheric lenses are used in cameras and binoculars, it would be wrong to assume that this means aspherics/atorics result in better optics for eyewear. Cameras and telescopes use multiple lens elements and have different design criteria. Spectacles are made of only one ophthalmic lens, and the best-form spheric lens has been shown to give the best vision.citation needed In cases where best-form is not used, such as cosmetic flattening, thinning or wrap-around sunglasses, an aspheric design can reduce the amount of induced optical distortions.citation needed

It is worth noting that aspheric lenses are a broad category. A lens is made of two curved surfaces, and an aspheric lens is a lens where one or both of those surfaces is not spherical. Further research and development is being conductedcitation needed to determine whether the mathematical and theoretical benefits of aspheric lenses can be implemented in practice in a way that results in better vision correction.

Optical aberrations of the eye lens vs. corrective lensedit

Optical terms are used to describe error in the eye's lens and the corrective lens. This can cause confusion since "astigmatism" or "ABBE" has drastically different impact on vision depending on which lens has the error.

Astigmatism disambiguationedit

Astigmatism of the eye: Patients who are prescribed a sphere and a cylinder prescription have astigmatism of the eye, and can be given a toric lens to correct it.

Astigmatism of the corrective lens: This phenomenon is called lens-induced oblique astigmatism error (OAE) or power error and is induced when the eye looks through the ophthalmic lens at a point oblique to the optical center (OC). This may become especially evident beyond -6D.

Example: A patient with astigmatism (or no astigmatism) of the eye and a high prescription may notice astigmatism of the lens (OAE) when looking through the corner of their glasses.

Aspheric and atoric disambiguationedit

In ophthalmic terminology, "aspheric lens" specifically refers to a subclass of aspheric lens. Designs that feature "flatter" curves trade optical quality for cosmetic appearance. By using a non-spheric lens shape, an aspheric lens attempts to correct the error induced by flattening the lens. Typically, the design focuses on reducing the error (OAE) across the horizontal and vertical lens axis edges. This is of primarily benefit to farsighted individuals, whose lenses have a thick center.

An atoric lens design refers to a lens with more complex aspheric lens design. An atoric lens design can address error over more corners of the lens, not just the horizontal and vertical axis.

A toric lens is designed to compensate for the astigmatism of a patient's eye. Even though this lens is technically "aspheric", the terms "aspheric" and "atoric" are reserved for lenses which correct errors induced by cosmetic lens flattening.

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