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.”

• All form of deviations are basically aberrations.

1. Chromatic2. Monochromatic Spherical Marginal Astigmatism Coma Curvature of Field Distortion

• 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).

• 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.

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

straight ahead through the lens.

– Off axis aberrations effect peripheral vision.

1) Low Degree Aberrations : It include Myopia, Hypermetropia & Regular Astigmatism. It can be surgically treated by conventional LASIK.

2) Higher Degree Aberrations : It include Spherical Aberration, Chromatic Aberration, Coma, Oblique Aberration, Centring, Diffraction of light & Distortion.

• Chromatic• Spherical• Marginal Astigmatism• Coma• Curvature of Field• Distortion

• Rays corresponding to different wavelengths travel different paths

• The lens material breaks white light into its component colors

1. Longitudinal Chromatic Aberration2. Lateral Chromatic Aberration

Blueimage

Redimage(Red ray)

(Blue Ray)

(Red ray)

(Blue Ray)

Longitudinalchromaticaberration

Object

Lateralchromaticaberration

Prismatic effectfor blue light

Prismatic effectfor red light

Lateral chromaticaberration

• It occurs when a prism bends light of two different wavelengths by different amounts.• Different image sizes• Result in colored ‘ghost’ images

White light

Lateral 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.

• 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.

Doublet lens

• Reducing chromatic aberration by Achromatic Aberration.

- Used two diff. Lens material one regular length an other occur correct the dispersion of 1st lens.

- One lens produced of crow glass i.e. low dispersion an other is flint glass i.e. high dispersion.

• Image is blurred or deformed due to the approximation error in the paraxial approximation to the exact solution.

• Aberrations can occur in a lens even when the light entering the lens is only one color.

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

Spherically aberratedfocal points

for peripheral rays

Focal Point for central (paraxial) rays

Spherical Aberration

Image is blurred or deformed due to the approximation error in the paraxial approximation to the exact solution

• Peripheral rays refract more strongly than paraxial rays.

• Correct with parabolic curves, aplanatic lens design.• Results in out-of-focus image.• Axial n Wide beam aberration – not important in

glasses design.• On-axis aberration.• Seidal aberration

i. Ant. Surface of cornea is flatter at paraxial than centre therefore act as aplanatic surface.

ii. The iris act as a stop to spherical aberration, impairment of visual acuity when pupil is dilated is due to spherical aberrations.

iii. Sph. Aberrations may reduced by occluding periphery of the lens that only the paraxial zone is used.

1st – Image quality es bcoz the focus us not stigmatic.

2nd – Image location change from the position.

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.

• The distance between the two line foci that occurs in oblique astigmatism is called the Astigmatic Difference.

• When expressed in diopters, this difference is called the Oblique Astigmatic Error.

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.

• Aplanatic Curvature of cornea n spherical surface of retina minimizing the effect of MA.

• It occur due to Sph. Lens show diff. refractive power in diff. meridian only when oblique rays fall upon them.

Correction for marginal (oblique)astigmatism, continued:

Object, way off to the left)

Image – cone or comet shaped.

• Type of Monochromatic aberration.• 2nd Seidal aberration similar to Sph.aberration.• Object point is off the axis of the lens, there is

a diff. in magnification for the rays passing through different zones of the lens.

• The composition image is not a circle bt elongated like Comet or Coma.

• Associated with off-axis object points.

• Not important in glasses design (except very high plus Rx).

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

problem when present.• Worst type of aberration• Degenerate and deforms the image of

point objects.

• Corrected with parabolic curves, aplanatic lens design.

• Reduce coma in high plus lens - Aspheric lens.• If head of coma points towards the optical axis

it is +ve coma.• If head of coma points away from the optical

axis it is –ve coma.

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 is the name for the curved

surface when marginal (oblique) astigmatism is correct.

• Another name for the Petzval's surface is the image sphere.

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

Distortion – pincushion – high plus lens

Object:

Distortion – barrel – high minus lens

Object:

• Another aberration of thick lenses.• In distortion, the object is sharply imaged bt

does not retain its shape.

• Two types of distortion;1.Barrel Distortion.2.Pincushion Distortion.

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

1. Barrel Distortion –

• Produced in minus lenses.• Rays in centre more magnified than the

further off-axis.• Due to minification of corners of a square

grid more from minus lenses.

2. Pincushion Distortion -

• Produced in plus lens.• Rays in centre are less magnified .• Due to the magnification of corners of square

object more from plus lens

• 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.

• Chromatic --------------- material dependent• Spherical (the rest are not)• Marginal Astigmatism• Coma• Curvature of Field• Distortion ----------------- in-focus image

(the rest give blurred images)

• Chromatic• Spherical Wide Beam• Marginal Astigmatism Narrow Beam• Coma Wide Beam• Curvature of Field Narrow Beam• Distortion

• Chromatic• Spherical On-axis• Marginal Astigmatism Off-axis• Coma Off-axis• Curvature of Field On-axis• Distortion

• Chromatic peripheral• Spherical central• Marginal Astigmatism peripheral

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

• Marginal Astigmatism• Curvature of Field . . . . . • Distortion• Chromatic aberration

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

A.K.Khurana, Theory and Practice of Optics and Refraction, 4th ed

A.K.Khurana, Comprehensive ophthalmology, 5th ed

Aberrations

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