Editing Corrective lens (section)

== Optical quality ==

=== 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.

=== Power error ===
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, the best spherical form lenses attempt to keep the ocular curve between four and seven diopters.

=== Lens induced oblique astigmatism ===
[[File:Astigmatism text blur.png|thumb|right|Effects of astigmatism]]
As the eye shifts its gaze from looking through the optical center of the corrective lens, the lens-induced [[astigmatism (optical systems)|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 does not have many parameters that can be traded off to improve vision. The index has little effect on error. 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. 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 sometimes 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.<ref name="Meister_lensdesign">{{cite web |url=http://www.opticampus.com/cecourse.php?url=lens_design/&OPTICAMP=f1e4252df70c63961503c46d0c8d8b60#asphericity |title=Ophthalmic Lens Design |first=Darryl |last=Meister |work=OptiCampus.com |access-date=November 12, 2008 |archive-url=https://archive.today/20130104185359/http://www.opticampus.com/cecourse.php?url=lens_design/&OPTICAMP=f1e4252df70c63961503c46d0c8d8b60%23asphericity |archive-date=January 4, 2013 |url-status=dead}}</ref>

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 manufacturers 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.
{{clear}}