Definitions

chirurgic

Corrective lens

A corrective lens is a lens worn in front of the eye, mainly used to treat myopia, hyperopia, astigmatism, and adjustable focus eyeglasses restore this ability to change focus by mimicking the lost elasticity of the eye’s crystalline lens, presbyopia. Glasses or "spectacles" are worn on the face a short distance in front of the eye. Contact lenses are worn directly on the surface of the eye. Intraocular lenses are surgically implanted most commonly after cataract removal, but recently for purely refractive purposes. Myopia (near sightedness) requires a divergent lens, whereas hyperopia (far sightedness) requires convergent lens.

Prescription of corrective lenses

Corrective lenses are typically prescribed by an optometrist. The prescription consists of all the specifications necessary to make the lens. Prescriptions typically include the power specifications of each lens (for each eye). Strengths are generally prescribed in quarter-diopter steps (0.25 D) because most people cannot generally distinguish between smaller increments (ex. eighth-diopter steps / 0.125 D).

Components of a sphero-cylindrical correction

Sphere component

Each power specification includes a spherical correction in diopters. Convergent powers are positive (ex. +4.00 D) and condense light to correct for farsightedness (hyperopia) or allow the patient to read more comfortably (see presbyopia and binocular vision disorders). Divergent powers are negative (ex. -3.75 D) and spread out light to correct for nearsightedness (myopia). If neither convergence nor divergence is required in the prescription, "plano" is used to denote a refractive power of zero.

Cylinder component

If a patient has an astigmatism, the patient needs two different correction powers in two different meridians (horizontally and vertically for example). This is specified by describing how the cylinder (the meridian that is most different from the spherical power) differs from the sphere power. Power evenly transitions between the two powers as you move from the meridian with the most convergence to the meridian with the least convergence or most divergence.

Ophthalmologists record in "plus cylinder notation" where the cylinder power is a number of diopters more convergent than the sphere power. That means the sphere power describes the most divergent meridian and the cylinder component describes the most convergent. Optometrists use "minus cylinder notation" where the cylinder power is a number of diopters more divergent than the sphere component. Thus the sphere power describes the most convergent meridian and the cylinder component describes the most divergent. (There is no difference in these forms of notation. They arise from the nature of the two professions and are easily converted between by people accustomed to working with sphero-cylindrical prescriptions. They are simply two ways to specify the same thing.)

Axis component

The axis defines where the two powers (sphere and cylinder) are located. The sphere is almost always 90º from the cylinder. (This is regular astigmatism, which is by far more common than irregular astigmatism where separations are other than 90º). Vertical is the 90th meridian and horizontal is both zero and the 180th meridians. The axis is the meridian 90º away from the cylinder power. Since the cylinder and sphere powers almost always separated by 90º, the axis is also the location of the sphere component. If the lens is spherical (there is no cylinder component) then there is no need for an axis. A prescription like this is written with D.S. (diopters sphere) after the spherical power.

Sample prescription

So, a prescription of -1.00 +0.25 x 180 describes a lens that has a horizontal power of -1.00 D and a vertical power of -0.75. (The same prescription written in minus cylinder notation: -0.75 - 0.25 x 090)

Other considerations

Single vision lenses correct for only distance or near vision. Patients with presbyopia or other disorders of accommodation often benefit prescriptions from corrections for both distance and near (see Lens Types below). Infrequently, prism and base curve values may also be specified to correct for binocular vision disorders.

Besides the use of lenses, chirurgic laser eye surgery (usually LASIK is also an option to correct a flawed vision; caused by the described disorders of accommodation). The main advantage is that no more lenses need to be bought/inserted and that in the long run, this option may be cheaper. Downsides are offcourse the chirurgic procedure itself.

Over the counter correction

In some cases, mild farsightedness (hyperopia) can be treated with simple magnifying lenses or commodity reading glasses that can be purchased without a prescription. The magnifiers make the image of the object being viewed bigger so that it can be seen better. Over the counter readers are spherical corrective lenses of varying strengths (commonly +1.00D to +4.00D).

These treatments are not as tailored to the specific needs of the patient. A difference in refractive error or presence of astigmatism will not be accounted for. The use of improper corrective lenses may not help or could even exacerbate binocular vision disorders. Over the counter readers may not work for patients with significant refractive errors requiring distance correction (unless they are used in combination with contact lenses that correct distance vision). Eyecare professionals (optometrists and ophthalmologists) are trained to determine the specific corrective lenses that will provide the clearest, most comfortable and most efficient vision, avoiding double vision and maximizing binocularity. They can tell patients if over the counter corrective lenses are appropriate.

Lens Types

Single Vision

Single vision has the same optical focal point or correction over the entire area of the lens.

Bifocal

Bifocal the upper part of the lens is generally used for distance vision, while the lower part is used for near vision. Usually, a segment line separates the two. Typically a person with myopia would have one section of a prescription lens that has a certain diverging power while another section of the lens would have a lower diverging power for close-up work. Similarly a person with hyperopia would have one section of the lens with a certain converging power and another section with a greater power for close-up work.

Trifocal

Trifocal similar to bifocals, except that the two focal areas are separated by a third middle area with intermediate focus correction, used for intermediate vision eg computer distance. This lens type has two segment lines, dividing the three different correcting segments.

Progressive

Progressive or varifocal provide a smooth transition from distance correction to near correction, eliminating segment lines and allowing the viewing of all intermediate distances.

Adjustable Focus

Adjustable or variable focus dynamically adjusts focal point to cleary view desired object and all distances. This allows clear focus from near to far and everywhere in between and is similar to having many single vision lenses enabling to see all distances clearly through the entire lens.

Lens shapes

Although corrective lenses can be produced in many different shapes, the most common is ophthalmic or convex-concave. In an ophthalmic lens, both the front and back surface have a positive radius, resulting in a positive / convergent front surface and a negative / divergent back surface. The difference in curvature between the front and rear surface leads to the corrective power of the lens. In hyperopia and presbyopia a convergent lens is needed, therefore the convergent front surface overpowers the divergent back surface. For myopia the opposite is true: the divergent back surface is greater in magnitude than the convergent front surface.

Bifocals and trifocals result in more complex lens shapes. A bifocal adds a second lens called an add segment to a standard distance corrective lens. There are many shapes and sizes of segments and the method that they are combined with the distance corrective lens has to do with the lens material and segment type (shape).

Progressive lenses, which eliminate the line in bi/tri-focals, are very complex in their shape as they are no longer the combination of two spherical surfaces.

The base curve (usually determined from the shape of the front surface of an ophthalmic lens) can be changed to result in the best optic and cosmetic characteristics across the entire surface of the lens. Optometrists may choose to specify a particular base curve when prescribing a corrective lens for either of these reasons. A multitude of mathematical formulas and professional clinical experience has allowed optometrists and lens designers to determine standard base curves that is ideal for most people.

Refractive index

In the UK and the US, the refractive index is generally specified with respect to the yellow He-d Fraunhofer line, commonly abbreviated as nd. Lens materials are classified by their refractive index, as follows:

  • Normal index - 1.48 ≤ nd < 1.54
  • Mid-index - 1.54 ≤ nd < 1.60
  • High-index - 1.60 ≤ nd < 1.74
  • Very high index - 1.74 ≤ nd

This is a general classification. Indexes of nd values that are ≥ 1.60 can be, often for marketing purposes, referred to as high-index. Likewise, Trivex and other borderline normal/mid-index materials, may be referred to as mid-index.

Advantages of higher indexes

  • Thinner, lighter lenses See Below
    • Convergent lenses (to correct hyperopia) of the same refractive power have a thinner center thickness when made from higher index.
    • In highly myopic cases high index can minimize edge thickness. This reduces light entering into the edge of the lens, reducing an additional source of internal reflections. This also makes the lenses more cosmetically appealing.
    • Since the lenses are made thinner, less material is used, making them lighter

Disadvantages of increased indices

  • Poorer Abbe number leading to increased chromatic aberration
  • Poorer light transmission and reflection on lens, according to the Fresnel reflection equation, increasing importance of anti-reflective coating.
  • Theoretically, manufacturing defects have more impact on optical quality.
  • Theoretically, off-axis optical quality degrades (oblique astigmatic error). This degradation should not be perceptible in practice.
    • Currently, frame styles are much smaller than they would have to be for these aberrations to be seen away from the optical center of the lens (off-axis).

Optical quality

Abbe number

Of all of the properties of a particular lens material, the one that most closely relates to its optical performance is its dispersion, which is specified by the Abbe number. Lower Abbe numbers result in the presence of chromatic aberration (i.e., color fringes above/below or to the left/right of a high contrast object), especially in larger lens sizes and stronger prescriptions (±4D or greater). Generally, lower Abbe numbers are a property of mid and higher index lenses that cannot be avoided, regardless of the material used. The Abbe number for a material at a particular refractive index formulation is usually specified as its Abbe value.

In practice, ABBE’s effect on chromatic aberration can be roughly estimated to change 1:1, meaning a change from 30 to 32 ABBE will not have a practically noticeable benefit, but a change from 30-47 could be beneficial for users with strong prescriptions that move their eyes and look ‘off-axis’ of optical center of the lens. Note that some users do not sense color fringing directly but will just describe 'off-axis blurriness'. Abbe values even as high as that of (Vd≤45) produce chromatic aberrations which can be perceptible to a user in lenses larger than 40mm in diameter and especially in strengths that are in excess of ±4D. At ±8D even glass (Vd≤58) produces chromatic aberration that can be noticed by a user. Chromatic aberration is independent of the lens being of spherical, aspheric, or atoric design.

The eye’s ABBE number is independent of the importance of the corrective lens’s ABBE, since the human eye:

  • Moves to keep the visual axis close to its achromatic axis, which is completely free of dispersion (i.e., to see the dispersion one would have to concentrate on points in the periphery of vision, where visual clarity is quite poor)
  • Is very insensitive, especially to color, in the periphery (i.e., at retinal points distant from the achromatic axis and thus not falling on the fovea, where the cone cells responsible for color vision are concentrated. See: Anatomy and Physiology of the Retina)

In contrast, the eye moves to look through various parts of a corrective lens as it shifts its gaze, some of which can be as much as several centimeters off of the optical center. Thus, despite the eye's ABBE properties, the corrective lens's ABBE value cannot be dismissed. People who are sensitive to the effects of chromatic aberrations, have stronger prescriptions, often look off the lens’s optical center, or prefer larger corrective lens sizes may be impacted by chromatic aberration. To minimize chromatic aberration, a doctor or wearer can:

  • Try to use the smallest vertical lens size that is comfortable. Generally, chromatic aberrations are more noticeable as the pupil moves vertically below the optical center of the lens (e.g., reading or looking at the ground while standing or walking). Keep in mind that a smaller vertical lens size will result in a greater amount of vertical head movement, especially while performing activities that involve short and intermediate distance viewing, which could lead to an increase in neck strain, especially in occupations involving a large vertical field of view.
  • Restrict the choice of lens material to the highest ABBE value at acceptable thickness. The oldest most basic commonly used lens materials also happen to have the best optical characteristics at the expense of corrective lens thickness (i.e., cosmetics). Newer materials have focused on improved cosmetics and increased impact safety, at the expense of optical quality. All lenses sold in USA pass the FDA ball-drop impact test, and depending on needed index these seem to currently have ‘best in class’ ABBE vs Index (Nd): Glass (2x weight of plastics) or CR-39 (2mm vs. 1.5mm thickness typical on newer materials) 58 @ 1.5, Sola Spectralite (47@1.53), Sola Finalite (43@1.6), and Hoya Eyry (36 @ 1.7). For impact resistance safety glass is offered at a variety of indexes at high ABBE, but is still 2x the weight of plastics. Polycarbonate (Vd=30-32) has very poor ABBE, but is tried and true. Trivex (Vd=43 @ 1.53), is also heavily marketed as an impact resistant alternative to Polycarbonate, for individuals who don’t need polycarbonate’s index. Trivex is also one of the lightest material available.
  • Use contact lenses in place of eyeglasses. A contact lens rests directly on the surface of the cornea and moves in sync with all eye movements. Consequently the contact lens is always directly aligned on center with the pupil and there is never any off-axis misalignment between the pupil and the optical center of the lens.

Power error (-D corrections for myopia)

Power error is the change in the optical power of a lens as the eye looks through various points on the area of the lens. Generally, it is least present at the optic center and gets progressively worse as one looks towards the edges of the lens. The actual amount of power error is highly dependent on the strength of the prescription as well as whether a best spherical form of lens or an optically optimal aspherical form was used in the manufacture of the lens. Generally, best spherical form lenses attempt to keep the ocular curve between four and seven diopters.

Lens induced oblique astigmatism (+D corrections for presbyopia)

As the eye shifts its gaze from looking through the optical center of the corrective lens, the lens induced astigmatism value increases. In a spherical lens, especially one with a strong correction whose base curve is not in the best spherical form, such increases can significantly impact the clarity of vision in the periphery.

Minimizing power error and lens induced astigmatism

As corrective power increases, even optimally designed lenses will have distortion that can be noticed by a user. This particularly affects individuals that use the off-axis areas of their lenses for visually demanding tasks. For individuals sensitive to lens errors, the best way to eliminate lens induced aberrations is to use contact lenses. Contacts eliminate all these aberrations since the lens then moves with the eye.

Barring contacts, a good lens designer doesn’t have many parameters which can be traded off to improve vision. Index has little effect on error. Note that, chromatic aberration is often perceived as ‘blurry vision’ in the lens periphery giving the impression of power error, although this is actually due to color shifting. Chromatic aberration can be improved by using a material with improved ABBE. The best way to combat lens induced power error is to limit the choice of corrective lens to one that is in the best spherical form. A lens designer determines the best-form spherical curve using the Oswalt curve on the Tscherning ellipse. This design gives the best achievable optical quality and least sensitivity to lens fitting. A flatter base-curve is sometime selected for cosmetic reasons. Aspheric or atoric design can reduce errors induced by using a suboptimal flatter base-curve. 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. The improvement due to flattening is most evident for strong farsighted lenses. High myopes (-6D) may see a slight cosmetic benefit with larger lenses. Mild prescriptions will have no perceptible benefit (-2D). Even at high prescriptions some high myope prescriptions with small lenses may not see any difference, since some aspheric lenses have a spherically designed center area for improved vision and fit.

In practice, labs tend to produce pre-finished and finished lenses in groups of narrow power ranges to reduce inventory. Lens powers that fall into the range of the prescriptions of each group share a constant base curve. 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. In this case the error will be imperceptible to the human eye. However, some manufacturer’s may further cost-reduce inventory and group over a larger range which will result in perceptible error for some users in the range who also use the off-axis area of their lens. Additionally some manufacturers may verge toward a slightly flatter curve. Although if only a slight bias toward plano is introduced it may be negligible cosmetically and optically. These optical degradations due to base-curve grouping also apply to aspherics since their shapes are intentionally flattened and then asphericized to minimize error for the average base curve in the grouping.

Cosmetics and weight

Reducing lens thickness

Note that the greatest cosmetic improvement on lens thickness (and weight) is had from choosing a frame which holds physically small lenses. The curves on the front and back of a lens are ideally formed with the specific radius of a sphere. This radius is set by the lens designer based on the prescription and cosmetic consideration. Selecting a smaller lens will mean less of this sphere surface is represented by the lens surface, meaning the lens will have a thinner edge (hyperopia) or center (myopia).

Index can improve the lens thinness, but at a point no more improvement will be realized. For example, if an index and lens size is selected with center to edge thickness difference of 1mm then changing index can only improve thickness by a fraction of this. This is also true with aspheric design lenses.

The lens minimum thickness can also be varied. The FDA ball drop test sets the minimum thickness of materials. Glass or CR-39 requires 2.0mm, but some newer materials only require 1.5mm or even 1.0mm minimum thickness.

Weight

Material density typically increases as lens thickness is reduced by increasing index. There is also a minimum lens thickness required to support the lens shape. These factors results in a thinner lens which is not lighter than the original. There are lens materials with lower density at higher index which can result in a truly lighter lens. These materials can be found a material property table. Reducing frame lens size will give the most noticeable improvement in weight for a given material.

Minification and magnification

Aspheric/atoric design can reduce minification and magnification of the eye for observers at some angles.

Lens materials

Optical crown glass (B270)

Glass lenses have become less common in recent years due to the danger of shattering and their relatively high weight compared to CR-39 plastic lenses. They still remain in use for specialised circumstances, for example in extremely high prescriptions (currently, glass lenses can be manufactured up to a refractive index of 1.9) and in certain occupations where the hard surface of glass offers more protection from sparks or shards of material. If the highest Abbe value is desired, the only choices for common lens optical material are optical crown glass and CR-39.

Higher quality optical-grade glass materials exist (e.g., Borosilicate crown glasses such as BK7 (nd=1.51680 / Vd=64.17 / D=2.51 g/cm³), which is commonly used in telescopes and binoculars, and fluorite crown glasses such as Schott N-FK51A (nd=1.48656 / Vd=84.47 / D=3.675 g/cm³), which is 16.2 times the price of a comparable amount of BK7, and are commonly used in high-end camera lenses). However, one would be very hard pressed to find a laboratory that would be willing to acquire or shape custom eyeglass lenses, considering that the order would most likely consist of just two different lenses, out of these materials. Generally, Vd values above 60 are of dubious value, except in combinations of extreme prescriptions, large lens sizes, a high wearer sensitivity to dispersion, and occupations that involve work with high contrast elements (e.g., reading dark print on very bright white paper, construction involving contrast of building elements against a cloudy white sky, a workplace with recessed can or other concentrated small area lighting, etc.).

Plastic (CR-39)

Plastic lenses are currently the most commonly prescribed lens, due to their relative safety, low cost, ease of production, and outstanding optical quality. The main drawbacks are the ease by which a lens can be scratched, and the limitations and costs of producing higher index lenses.

Trivex

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 value) and a slightly lower density. Its lower refractive index of 1.532 vs. polycarbonate's 1.586, however, may result in slightly thicker lenses. Along with polycarbonate and the various high-index plastics, Trivex is a lab favorite for use in rimless frames, due to the ease with which it can be drilled as well as its resistance to cracking around the drill holes. One other advantage that Trivex has over polycarbonate is that it can be easily tinted.

Polycarbonate

Lighter weight than normal plastic. Less tendency to irritate your nose or leave red marks on your nose where the glasses touch your nose. Polycarbonate blocks UV rays, is shatter resistant and is used in sports glasses and glasses for children and teenagers. Because polycarbonate is soft and will scratch easily, scratch resistant coating is typically applied after shaping and polishing the lens. Standard polycarbonate with an Abbe value of 30 is one of the worst materials optically, if chromatic aberration intolerance is of concern. Along with Trivex and the high-index plastics, polycarbonate is an excellent choice for rimless eyeglasses. Similar to the high-index plastics, polycarbonate has a very low Abbe value which may be bothersome to individuals sensitive to chromatic aberrations.

High-index plastics (polyurethanes)

High-index plastics allow for thinner lenses. The lenses may not be lighter, however, due to the increase in density vs. mid- and normal index materials. Despite being popular with customers, due their thinner appearance, high-index lenses also suffer from a much higher level of chromatic aberrations due to their lower Abbe value. For people with strong prescriptions, the significant reduction in thickness may warrant the reduction in optical quality. Aside from thinness of the lens, another advantage of high-index plastics is their strength and shatter resistance, although not as shatter resistant as polycarbonate. This makes them another excellent choice for rimless eyeglasses.

Ophthalmic material property tables

Material, Plastic Index (Nd) ABBE (Vd) Specific Gravity UVB/ UVA Reflected light (%) Minimum thickness typ/min (mm) Note
Next Gen Transitions 1.50 58 1.27 100% / 100% 7.92
CR-39 Hard Resin 1.50 58 1.32 100% / 90% 7.97 ?/2.0
PPG Trivex (Average) 1.53 44 1.11 100% / 100% 8.70 ?/1.0 PPG, Augen, HOYA, Thai Optical, X-cel, Younger
SOLA Spectralite 1.54 47 1.21 100% / 98% 8.96 (also Vision 3456 (Kodak)?)
Essilor Ormex 1.56 37 1.23 100% / 100% 9.52
Polycarbonate 1.59 30 1.20 100% / 100% 10.27 ?/1.5 Tegra (Vision-Ease) Airwear (Essilor) FeatherWates (LensCrafters)
MR-8 1.6 Plastic 1.6 41 1.30 100% / 100% 10.43
MR-6 1.6 Plastic 1.6 36 1.34 100% / 100% 10.57
SOLA Finalite 1.60 42 1.22 100% / 100% 10.65
MR-7 1.67 Plastic 1.66 32 1.35 100% / 100% 12.26
MR-10 1.67 Plastic 1.66 32 1.37 100% / 100% 12.34
Hoya EYRY 1.70 36 1.41 100% / 100% 13.44 ?/1.5
MR-174 1.74 Plastic 1.73 33 1.47 100% / 100% 14.36 Hyperindex 174 (Optima)

Material, Glass Index (Nd) ABBE (Vd) Specific Gravity UVB/ UVA Reflected light (%) Minimum thickness typ/min (mm) Note
Crown Glass 1.525 59 2.54 79% / 20% 8.59
PhotoGray Extra 1.523 57 2.41 100% / 97% 8.59
1.6 Glass 1.604 40 2.62 100% / 61% 10.68 Zeiss Uropal, VisionEase, X-Cel
1.7 Glass 1.706 30 2.93 100% / 76% 13.47 Zeiss Tital, X-Cel, VisionEase, Phillips
1.8 Glass 1.800 25 3.37 100% / 81% 16.47 Zeiss Tital, X-Cell, Phillips, VisionEase
1.9 Glass 1.893 31 4.02 100% / 76% 18.85 Zeiss Lantal
Reflected light calculated using Fresnel reflection equation for normal waves against air on two interfaces. This is reflection without an AR coating.

Compilations of manufacturer material data can be found at opticampus, firstvisionmedia, and eyecarecontacts Additional information on branding can be found at eyetopics

Indices of refraction for a range of materials can be found in the List of indices of refraction.

Lens coatings

Anti-reflective

Anti-reflective coatings help to make the eye behind the lens more visible. 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). Such reduction of back reflections increases the apparent contrast of surroundings. At night, anti-reflective coatings help to reduce headlight glare from oncoming cars, street lamps and heavily lit or neon signs.

One problem with anti-reflective coatings is that historically they have been very easy to scratch. Newer coatings, such as Crizal Alizé with its 5.0 rating and Hoya's Super HiVision with its 10.9 rating on the COLTS Bayer Abrasion Test (glass averages 12-14), try to address this problem by combining scratch resistance with the anti-reflective coating. They also offer a measure of dirt and smudge resistance, due to their hydrophobic properties (110° water drop contact angle for Super HiVision and 112° for Crizal Alizé).

Ultraviolet protection

A UV coating is used to reduce the transmission of light in the ultraviolet spectrum. UV-B radiation increases the likelihood of cataracts, while long term exposure to UV-A radiation can damage the retina. DNA damage from UV light is cumulative and irreversible. Some materials, such as Trivex and Polycarbonate naturally block most UV light and do not benefit from the application of a UV coating.

Scratch resistance

Highly recommended, especially for polycarbonate and softer materials, to make lenses last longer. This is done automatically by many labs for polycarbonate and high index lenses.

Confusing corrective lens industry terminology

Spheric vs. aspheric, atoric, etc.

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". This qualification is necessary since best-form spherics are always better than aspherics for an ophthalmic lens application. Aspherics for corrective lenses are only used to attempt to improve the degradation caused by deviating from best-form sphere resulting from making a flatter lens for cosmetic reasons. The same applies for atoric and bi-aspheric.

It is true that aspheric lenses are used in cameras and binoculars. It would be wrong to assume that this means aspherics/atorics are a sign of good optics in eyewear. Cameras and telescopes use multiple lens elements and have different design criteria. Spectacles are made of only one ophthalmic lens. The best-form spheric lens has been shown to give the best vision. 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.

The problem with aspheric lenses is that they 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 conducted to determine if the mathematical and theoretical benefits of aspheric lenses can actually lead to better vision correction.

Optical aberrations of the eye lens vs. corrective lens

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 disambiguation

Astigmatism of the eye: People 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 disambiguation

An ophthalmic "aspheric lens" specifically refers to a subclass of aspheric lens. Designs referring to "flatter" curves are trading off optical quality for cosmetic appearance. An aspheric lens attempts to correct the error induced by flattening the lens by using a non-spheric lens shape. Typically the design focuses on reducing the error (OAE) across the horizontal and vertical lens axis edges. This can be primarily beneficial 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" (two words, not "atoric") lens is a lens designed to compensate for the patients with astigmatism in their eye. Even though this technically requires an "aspheric" lens, "aspheric" and "atoric" are reserved for lenses which correct errors induced by cosmetic lens flattening.

See also

References

External links

Lens type

Adjustable focus

Lens materials

Lens coatings

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