Corrective lens

Another criticism of over-the-counter glasses is that they may alleviate symptoms, causing a person to forgo the other benefits of routine vision exams, such as early diagnosis of chronic disease.

Bifocals allow people with presbyopia to see clearly at distance and near without having to remove the glasses, which would be required with single vision correction.

[3] The lack of any abrupt change in power and the uniform appearance of the lens gives rise to the name "no-line bifocal".

The need for constant adjustment when the person's attention switches to an object at a different distance is a design challenge of such a lens.

Some people with good natural eyesight like to wear eyeglasses as a style accessory, or want to change the appearance of their eyes using novelty contact lenses.

A multitude of mathematical formulas and professional clinical experience has allowed optometrists and lens designers to determine standard base curves that are ideal for most people.

Steeper and more convergent than the base curve, the add segment combines with the back surface to deliver the person's near correction.

Some "seg type" examples include Flat top, Kryptok, Orthogon, Tillyer Executive, and Ultex A. Trifocals contain two add segments to achieve a lens that corrects the person's vision for three distinct distances.

In the UK and the US, the refractive index is generally specified concerning the yellow He-d Fraunhofer line, commonly abbreviated as nd.

As the eye shifts its gaze from looking through the optical center of the corrective lens, the lens-induced astigmatism value increases.

Note that, although chromatic aberration is often perceived as "blurry vision" in the lens periphery and gives the impression of power error, this is actually due to color shifting.

They cannot surpass the optical quality of a spherical best-form lens but can reduce the error induced by using a flatter-than-optimal base curve.

[5] In practice, labs tend to produce pre-finished and finished lenses in groups of narrow power ranges to reduce inventory.

For example, corrections from -4.00D to -4.50D may be grouped and forced to share the same base curve characteristics, but the spherical form is only best for a -4.25D prescription.

The greatest cosmetic improvement on lens thickness (and weight) benefits from choosing a frame that holds physically small lenses.

Ways to reduce the weight and thickness of corrective lenses, in approximate order of importance are these: It is not always possible to follow the above points, because of the rarity of such frames, and the need for a more pleasing appearance.

One must bear in mind that the human eye itself has an Abbe number Vd ≈ 50.2 so the expensive, high-end optical glass types mentioned above have little value for central vision; however, the wearer's view through the side of a glass lens is not comparable to central vision through the eye: Low dispersion glass definitely makes optically superior corrective lenses since it greatly reduces color-fringing of edge-wise viewed contrasty objects, compared to all available plastics.

Also, many exotic glass types, with Abbe number Vd ≳ 65 , contain oxides of heavy metals such as arsenic or lanthanum, some of which are toxic.

Abbe numbers (Vd) in excess of Crown Glass and CR-39 are mainly warranted only for unusual special uses, such as Plastic lenses are currently the most commonly prescribed lens, owing to their relative safety, low cost, ease of production, and high optical quality.

[8] PPG named the material Trivex because of its three main performance properties: Superior optics, ultra lightweight, and extreme strength.

[9] Trivex is a relative newcomer that possesses the UV-blocking properties and shatter resistance of polycarbonate while at the same time offering far superior optical quality (i.e., higher Abbe number) and a slightly lower density.

Because polycarbonate is soft and will scratch easily, a scratch-resistant coating is typically applied after shaping and polishing the lens.

Similar to the high-index plastics, polycarbonate has a very low Abbe number, which may be bothersome to individuals sensitive to chromatic aberrations.

These high-refractive-index plastics are typically thiourethanes, with the sulfur atoms in the polymer being responsible for the high refractive index.

They also help lessen back reflections of the white of the eye as well as bright objects behind the eyeglasses wearer (e.g. windows, lamps).

At night, anti-reflective coatings help to reduce headlight glare from oncoming cars, street lamps, and heavily lit or neon signs.

[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.

Further research and development is being conducted[citation 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.

This can cause confusion since "astigmatism" or "ABBE" has a drastically different impact on vision depending on which lens has the error.

Typically, the design focuses on reducing the error (OAE) across the horizontal and vertical lens axis edges.