Optical Theory II ABERRATIONS

Copyright 2001 -- Ellen Stoner, MALS, ABOM, NCLC

Aberrations

“When light from a point source goes through a correctly powered spectacle lens yet fails to create a perfect image, the cause is lens aberration.”

Brooks & Borish, Systems for Ophthalmic Dispensing, 2nd edition,

page 501

Classifications of Aberrations

• Chromatic vs. Monochromatic – Depends on the material of the lens– Requires the beam of light to contain more than

one wavelength

Classifications of Aberrations

• In Focus vs. Out of Focus – Out of focus aberrations cause fuzzy images

where clear sharp images should be– In focus aberrations cause images to be the

wrong shape (distorted).

Classifications of Aberrations

• Wide Beam vs. Narrow Beam – Wide beam aberrations are not as important

when the light goes through a narrow opening or aperture, such as the pupil of the eye.

– Narrow beam aberrations are the important aberrations when making glasses.

– Wide beam aberrations are important for optical instruments such as telescopes.

Classifications of Aberrations

• On Axis vs. Off Axis – On axis aberrations effect vision when looking

straight ahead through the lens.– Off axis aberrations effect peripheral vision.

Lens Aberrations

• Chromatic

• Spherical

• Marginal Astigmatism

• Coma

• Curvature of Field

• Distortion

• The lens material breaks white light into its component colors

• Why? Index of refraction varies by wavelength.

Chromatic Aberration

Longitudinal (axial)• The placement of the various focal points on the axis.

• This is the source of the Abbé value

Lateral (magnification)• Different image sizes

• Result in colored ‘ghost’ images

Chromatic Aberration

• Material dependent.

• Results in out of focus image.

• wearer complains of peripheral color fringes(more pronounced off-axis).

• The higher the power of the lens, the more the chromatic aberration.

Chromatic Aberration

• Abbé value : High Abbé, low aberration

Low Abbé, high aberration

Relation between index of refraction and Abbé value is not perfect. Within a material classification it works somewhat: for example, comparing types of glass: change of index resulting from different amounts of barium in the glass.

Chromatic Aberration Abbé value index

Crown glass 58 1.523

CR-39 58 1.498

PGX 57 1.523

Spectralite 47 1.537

1.6 PGX 42 1.60

Polycarbonate 30 1.586

Brooks & Borish, Systems for Ophthalmic Dispensing 2nd ed., page 503

Chromatic Aberration• Correction:

Doublet lens (for instruments: cameras, telescopes, microscopes).

Change lens materials. AR coat. Careful placement of OC’s:

Monocular PD;OC height and pantoscopic tilt;Short vertex distance and small frame;Control edge thickness.

Consumer education.

Lens Aberrations

• Chromatic

• Spherical

• Marginal Astigmatism

• Coma

• Curvature of Field

• Distortion

Spherical lens:Peripheral rays have shorterfocal length than paraxial rays.

• Peripheral rays refract more than paraxial rays.

• Correct with parabolic curves, aplanatic lens design.

• Results in out-of-focus image.• Wide beam aberration – not important in glasses

design.• On-axis aberration.

Lens Aberrations

• Chromatic

• Spherical

• Marginal Astigmatism

• Coma

• Curvature of Field

• Distortion

Spherical lens, narrow beamentering off-axis.

• Narrow beam aberration, therefore important in glasses lens design.

• Beam enters obliquely to lens axis, therefore effects peripheral vision.

• Creates excess power and cylinder• Also called Oblique astigmatism or Radial

astigmatism.• Correct Curve lens design for glasses corrects for

this aberration.

• Tscherning’s ellipse – a graph showing the best base curve for every Rx, to minimize marginal (oblique) astigmatism.

• Goes from about –23D to about +7D. Outside that range there is no ‘perfect’ base curve.

• Tscherning’s ellipse gives two correct base curves: one in the pl to +12 range, one in a higher plus power. We traditionally use the lower one.

Correction for Marginal (oblique) astigmatism, continued:

• Pantoscopic tilt / OC height combination.– Lower OC 1 mm for every 2 degrees

pantoscopic tilt.– Use face form in glasses where the OC’s are

decentered in.

• Aspheric design for high powers and large lenses.

Lens Aberrations

• Chromatic

• Spherical

• Marginal Astigmatism

• Coma

• Curvature of Field

• Distortion

Object, way off to the left)

Image – cone or comet shaped.

• Wide beam aberration, so not important in glasses design (except very high plus Rx).

• Corrected with parabolic curves, aplanatic lens design.

• Results in out-of-focus image.• Off-axis aberration, so a peripheral vision

problem when present.• For very high plus lenses, aspheric designs

will improve coma.

Lens Aberrations

• Chromatic

• Spherical

• Marginal Astigmatism

• Coma

• Curvature of Field

• Distortion

Plane of focus when Marginal astigmatism is corrected

Plane of focus when Curvature of field is corrected

• Also called power error.• Light does not focus on a flat focal plane.

The focal plane is curved.• Remember the screens at drive-in movies?

They are curved, not flat, to focus the sides of the movie as well as the center.

• The retina at the back of your eye globe is not a flat plane. It is curved.

• Curvature of field is minimized with corrected curve design base curves.

• This aberration effects peripheral vision.• Petzval’s surface, or the image sphere is

the name for the curved surface when marginal (oblique) astigmatism is correct.

• Far point sphere is where the image would focus correctly.

Lens Aberrations

• Chromatic

• Spherical

• Marginal Astigmatism

• Coma

• Curvature of Field

• Distortion

Distortion

Distortion – pincushion – high plus lens

Object:

Distortion

Distortion – barrel – high minus lens

Object:

Distortion

Brooks Systems for Ophthalmic lens Work, 2nd ed, page 509

Distortion

• Image is in focus, but not shaped the same as the object.

• Results from increased prism away from the OC of the lens.

• Solution is aspheric design lenses.

• Minor importance for glasses lenses.

Lens Aberrations

• Chromatic --------------- material dependent

• Spherical (the rest are not)

• Marginal Astigmatism

• Coma

• Curvature of Field

• Distortion ----------------- in-focus image (the rest give blurred images)

Lens Aberrations

• Chromatic

• Spherical wide beam

• Marginal Astigmatism narrow beam

• Coma wide beam

• Curvature of Field narrow beam

• Distortion

Lens Aberrations

• Chromatic

• Spherical on-axis

• Marginal Astigmatism off-axis

• Coma off-axis

• Curvature of Field on-axis

• Distortion

Lens Aberrations

• Chromatic peripheral• Spherical central• Marginal Astigmatism peripheral

(Central when pantoscopic tilt incorrect)• Coma peripheral• Curvature of Field peripheral• Distortion peripheral

Lens AberrationsIn order of importance for lens

design:

• Marginal Astigmatism

• Curvature of Field

. . . . .

• Distortion

• Chromatic aberration

References

Brooks & Borish, System for Ophthalmic Dispensing, 2nd ed, Butterworth-Heinemann, 1996.

Meyer-Arendt, Introduction to Classical and Modern Optics, 4th ed, Prentice-Hall, 1995.