Editing Corrective lens (section)

=== Abbe number ===
{{main article|Abbe number}}
[[File:Chromatic aberration convex.svg|thumb|right|Chromatic aberration caused by a convex lens]]
[[File:Nearsighted color fringing -9.5 diopter - Canon PowerShot A640 thru glasses - overview.jpg|thumb|right|Prismatic color distortion shown with a camera set for nearsighted focus, and using -9.5 diopter eyeglasses to correct the camera's myopia.]]

[[File:Nearsighted color fringing -9.5 diopter - Canon PowerShot A640 thru glasses - closeup detail.jpg|thumb|right|Close-up of color shifting through corner of an eyeglass lens. The visible colored fringing along the light and dark borders between color swatches are not actually on the color chart: They are the result of dispersion of colors by the lens.]]

Of all of the properties of particular lens material, the one that most closely relates to its optical performance is its [[Dispersion (optics)|dispersion]], which is specified by the [[Abbe number]].  Higher Abbe numbers mean a better lens material, and 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 (beyond ±4.00[[dioptre|D]]). 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, an Abbe number change from 30 to 32 will not have a practically noticeable benefit, but a change from 30 to 47 could be beneficial for users with strong prescriptions that move their eyes and look "off-axis" of the optical center of the lens.{{Citation needed|date=August 2013}} Some users do not sense color fringing directly but will just describe "off-axis blurriness".{{Citation needed|date=August 2013}}
Abbe values even as high as that of {{nobr|( {{mvar|V}}{{sub|d}} ≤ 45 )}} produce chromatic aberrations which can be perceptible to a user in lenses larger than 40&nbsp;mm in diameter and especially in strengths that are in excess of ±4&nbsp;D. At ±8&nbsp;D even glass {{nobr|( {{mvar|V}}{{sub|d}} ≤ 58 )}} produces chromatic aberration that can be noticed by a user.{{citation needed|date=August 2013}} 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 [[retina]]l points distant from the achromatic axis and thus not falling on the [[Fovea centralis|fovea]], where the [[cone cell]]s responsible for color vision are concentrated. ''See: [https://web.archive.org/web/20120505103132/http://www.psych.ndsu.nodak.edu/mccourt/Psy460/Anatomy%20and%20physiology%20of%20the%20retina/Anatomy%20and%20physiology%20of%20the%20retina.html 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 dispersive properties, the corrective lens's dispersion cannot be dismissed. People who are sensitive to the effects of chromatic aberrations, or who have stronger prescriptions, or who often look off the lens's optical center, or who prefer larger corrective lens sizes may be impacted by chromatic aberration. To minimize chromatic aberration:
* 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 an 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. Lenses sold in the US must pass the [[Food and Drug Administration]] ball-drop impact test, and depending on needed index these seem to currently have "best in class" Abbe vs Index {{nobr|( {{mvar|N}}{{sub|d}} ):}} Glass (2× weight of plastics) or [[CR-39]] (2&nbsp;mm vs. 1.5&nbsp;mm 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 number, but is still 2x the weight of plastics. Polycarbonate {{nobr|( {{mvar|V}}{{sub|d}} {{=}} 30–32 )}} is very dispersive but has excellent shatter resistance. Trivex {{nobr|( {{mvar|V}}{{sub|d}} {{=}} 43 @ 1.53 ),}} is also heavily marketed as an impact-resistant alternative to Polycarbonate, for individuals who do not need polycarbonate's index. Trivex is also one of the lightest materials available.
* Use contact lenses in place of or as well as eyeglasses. A contact lens rests directly on the surface of the cornea and moves in sync with all eye movements; consequently, a contact lens is always almost perfectly aligned on center with the pupil, and there is never any significant off-axis misalignment between the pupil and the optical center of the lens.