3.3 Aberrations, Polarization & Absorptive Optics

The Lens Aberrations

A perfect lens would image a point as a point. Real lenses introduce aberrations that blur or distort the image, and the ABO Advanced exam expects you to identify each and know its cure.

Chromatic aberration comes from dispersion: a material bends short (blue) wavelengths more than long (red) ones, splitting white light into colored fringes. Its magnitude is captured by the Abbe value (constringence) — higher Abbe means less dispersion. A practical estimate of transverse chromatic aberration (TCA) at any point is:

TCA = prismatic effect / Abbe value

Look 10 mm (1 cm) off-center through a +5.00 D lens: the prism is 1 x 5 = 5 prism diopters. In polycarbonate (Abbe 30) the color spread is 5 / 30 = 0.17 prism diopters; in CR-39 (Abbe 58) it is 5 / 58 = 0.09 prism diopters — nearly half. That is why a high-minus polycarbonate wearer reports rainbow fringes around night lights while CR-39 or Trivex (Abbe 43-45) does not.

The remaining four are the monochromatic (Seidel) aberrations:

• Spherical aberration — peripheral rays focus in front of paraxial rays; softens on-axis focus, mostly relevant in high plus.
• Coma — an off-axis point images as a comet or teardrop because zones magnify unequally.
• Oblique (radial) astigmatism — off-axis rays form two focal lines; the dominant spectacle aberration, controlled by base curve.
• Curvature of field — the sharp image forms on a curved surface, not the flat retina.
• Distortion — magnification varies with field angle, giving pincushion (plus) or barrel (minus) shapes; most visible in high powers.

Corrected-Curve (Best-Form) Lenses

Corrected-curve or best-form lenses are designed on Tscherning's ellipse, the curve of front base powers that eliminates oblique astigmatism for a given prescription. The ellipse offers two solutions per power: the steep, bulbous Wollaston branch and the flatter, cosmetically superior Ostwalt branch, which nearly all modern stock lenses use. A point-focal design zeroes oblique astigmatism, while a Percival form instead minimizes the mean power (spherical) error. Free-form digital surfacing now achieves best-form correction on the back surface for any base curve, freeing the designer from the small set of pre-molded blanks. Understanding this is why you cannot flatten a lens arbitrarily: leave the ellipse and peripheral vision degrades.

Polarization and Polarized Lenses

Light reflecting off a flat horizontal surface — water, wet roads, snow, a car hood — becomes strongly horizontally polarized, producing harsh glare. A polarized lens contains a stretched polyvinyl alcohol (PVA) film whose long dye molecules are aligned so the lens transmission axis is vertical. It therefore absorbs horizontally polarized light and passes the rest, cutting reflected glare without simply dimming the whole scene the way a plain tint does.

Because the filter is orientation-specific, mounting matters: the film must be aligned at 180 during fabrication, or glare rejection fails. Polarized lenses can also darken or blank some LCD and dashboard displays and make tempered-glass stress patterns visible, points to counsel patients about. Polarization is a filtering mechanism and is entirely separate from photochromic darkening, which is a UV-triggered chemical reaction; a lens can be one, the other, both, or neither.

Absorptive Lenses, UV, and Tints

Absorptive lenses attenuate specific wavelengths. The ultraviolet band is divided into UV-C (100-280 nm), absorbed by the atmosphere; UV-B (280-315 nm), linked to photokeratitis, pterygium, and cortical cataract; and UV-A (315-400 nm), linked to lens and retinal damage. Quality lenses block up to UV400, meaning 100 percent absorption to 400 nm. Polycarbonate and Trivex block UV inherently; CR-39 needs a UV additive or coating.

Visible light transmittance (VLT) defines sunglass categories under ANSI Z80.3 and ISO 12312-1:

Tint color changes contrast, not just darkness. Gray is neutral and preserves color balance; brown/amber absorbs blue light to heighten contrast and depth (good for variable light and sport); green (G-15) transmits about 15 percent with fairly true color; yellow boosts contrast in low light but distorts color; rose/FL-41 eases light sensitivity for some migraine patients. Plastic lenses take tint by immersion in a heated dye bath, while glass is colored by metal-oxide additives fired into the melt and cannot be surface dip-dyed.

Gradient tints run dark at the top fading to clear at the bottom (dip-dyed by controlling immersion depth), useful for driving without dimming the dashboard, whereas double-gradient tints are dark top and bottom with a lighter middle. A mirror coating is a reflective metallic layer that lowers transmittance further and is cosmetic plus glare-reducing, but it does not by itself provide UV protection. Blue-light filtering lenses attenuate the roughly 400-455 nm high-energy visible band; they are marketed for digital eye strain, though the strongest evidence remains for UV, not visible blue.

Common Traps

• Abbe governs color, not thinness. A thin 1.74 lens (Abbe 33) disperses more than thick CR-39 (Abbe 58); high index buys thinness at an optical cost.
• Polarized and photochromic are different mechanisms — one filters horizontally polarized glare, the other darkens with UV; do not assume a dark lens is polarized.
• A dark tint is not UV protection. Transmittance category rates visible light only; verify UV400 separately, since a dark, non-UV lens dilates the pupil and admits more UV.
• Category 4 lenses are not legal for driving because they transmit only 3-8 percent of visible light.