Aberrations stop even a correctly powered lens from producing a perfect image. Chromatic aberration separates colors. The five monochromatic kinds are coma, distortion, oblique astigmatism, spherical aberration, and curvature of field. Tilt a spherical lens and it picks up unwanted cylinder matching its own sign, along with a small sphere increase.
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What an aberration is
An aberration is an image flaw that remains even when the lens has the right power. They fall into two families: chromatic, which depends on wavelength, and monochromatic, which shows up even in single-color light.
Chromatic aberration
Each wavelength bends by a slightly different amount, so colors come to focus at different spots. Brooks and Borish note that this can leave an image with a colored fringe. Material choice and Abbe value are covered in lens materials and Abbe value; compare materials with the lens materials comparison.
The five Seidel aberrations
These are the monochromatic group, one at a time.
Coma
A point off the lens axis turns into a smeared, comet-shaped blur.
Spherical aberration
Rays through the lens edge focus at a different point from rays through the center. A wider pupil or aperture lets in more edge rays, so there is more of it.
Oblique astigmatism
Light crossing the lens obliquely forms two line foci at different distances rather than one point.
Curvature of field
One of the five Seidel aberrations.
Distortion
Magnification changes from center to edge, bending straight lines. Plus lenses give pincushion (corners pulled out); minus lenses give barrel (corners pulled in).
Lens tilt and induced cylinder
When a spherical lens is tilted, light meets it obliquely, so it acts like a sphere plus a cylinder. The induced cylinder carries the lens’s own sign and lines up with the axis of tilt, which is horizontal (axis 180) for pantoscopic tilt and vertical (axis 90) for wrap. A small sphere increase of the same sign comes with it. For frame fitting, see pantoscopic tilt and frame fit; for power along a diagonal meridian, see oblique meridian power.
Original practice set
Original problems.
Brooks and Borish describe an aberration that gives images a colored fringe. Chromatic or monochromatic?
Show answer
Chromatic.
A square grid viewed through strong plus power bows outward at the corners. Name the distortion.
Show answer
Pincushion.
A −6.00 D sphere is tilted pantoscopically. What sign and axis does the cylinder it picks up have?
Show answer
Minus cylinder, axis 180.
A plus sphere is tilted about the vertical meridian by wrap. Induced cylinder axis?
Show answer
Axis 90, the axis of tilt.
Rays passing near a lens edge focus at a different point from rays through its center. Which Seidel aberration is this?
Show answer
Spherical aberration.
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Frequently asked questions
What are lens aberrations?
Ways a real lens fails to bring light from a point to a single sharp point image, even when its power is right. They split into chromatic and monochromatic groups.
Sources: Brooks & Borish, System for Ophthalmic Dispensing, 3rd ed. (textbook), ch. 18 Lens Design, pp. 402–403, 409–411, 414 (opens in a new tab) ; University of Iowa EyeRounds — Wilkinson & Shahid, Optics Review (PDF), §20 pp. 42, 45–47 (opens in a new tab)
What is the difference between chromatic and monochromatic aberration?
Chromatic aberration happens because different wavelengths focus at different places; Brooks and Borish note it can give an image a colored fringe. Monochromatic aberrations occur even with light of one wavelength.
Sources: Brooks & Borish, System for Ophthalmic Dispensing, 3rd ed. (textbook), ch. 18 Lens Design, pp. 402–403, 409–411, 414 (opens in a new tab) ; University of Iowa EyeRounds — Wilkinson & Shahid, Optics Review (PDF), §20 pp. 42, 45–47 (opens in a new tab)
What are the five Seidel aberrations?
Coma, distortion, oblique astigmatism, spherical aberration, and curvature of field.
Sources: Brooks & Borish, System for Ophthalmic Dispensing, 3rd ed. (textbook), ch. 18 Lens Design, pp. 402–403, 409–411, 414 (opens in a new tab) ; University of Iowa EyeRounds — Wilkinson & Shahid, Optics Review (PDF), §20 pp. 42, 45–47 (opens in a new tab)
What is oblique astigmatism?
When light crosses a lens obliquely, it focuses as two separate line images instead of one point.
Sources: Brooks & Borish, System for Ophthalmic Dispensing, 3rd ed. (textbook), ch. 18 Lens Design, pp. 402–403, 409–411, 414 (opens in a new tab) ; University of Iowa EyeRounds — Wilkinson & Shahid, Optics Review (PDF), §20 pp. 42, 45–47 (opens in a new tab)
Do plus lenses cause pincushion or barrel distortion?
Plus lenses cause pincushion distortion; minus lenses cause barrel distortion.
Sources: Brooks & Borish, System for Ophthalmic Dispensing, 3rd ed. (textbook), ch. 18 Lens Design, pp. 402–403, 409–411, 414 (opens in a new tab) ; University of Iowa EyeRounds — Wilkinson & Shahid, Optics Review (PDF), §20 pp. 42, 45–47 (opens in a new tab)
What happens when a spherical lens is tilted?
It behaves like a sphere-cylinder lens. The induced cylinder has the same sign as the lens, and a little extra sphere of that sign appears too.
Sources: Brooks & Borish, System for Ophthalmic Dispensing, 3rd ed. (textbook), ch. 18 Lens Design, pp. 402–403, 409–411, 414 (opens in a new tab) ; University of Iowa EyeRounds — Wilkinson & Shahid, Optics Review (PDF), §20 pp. 42, 45–47 (opens in a new tab)
Which way is the cylinder axis when a lens has pantoscopic tilt?
Along the axis of tilt. Pantoscopic tilt rotates the lens about a horizontal line, so the induced cylinder sits at axis 180.
Sources: Brooks & Borish, System for Ophthalmic Dispensing, 3rd ed. (textbook), ch. 18 Lens Design, pp. 402–403, 409–411, 414 (opens in a new tab) ; University of Iowa EyeRounds — Wilkinson & Shahid, Optics Review (PDF), §20 pp. 42, 45–47 (opens in a new tab)
Official sources
- Brooks & Borish, System for Ophthalmic Dispensing, 3rd ed. (textbook), ch. 18 Lens Design, pp. 402–403, 409–411, 414 (opens in a new tab)
- University of Iowa EyeRounds — Wilkinson & Shahid, Optics Review (PDF), §20 pp. 42, 45–47 (opens in a new tab)
Last reviewed . Requirements, fees, and dates change — always confirm with the official source before you register or apply. ABO-NCLE and IJCAHPO are the only authorities for their own exam facts, so those facts rest on a single source.