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