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Aberrations of Lenses
Aberrations of Lenses
Aberrations of Lenses
Aberrations of Lenses
Aberrations of Lenses
Aberrations of Lenses
Aberrations of Lenses
Aberrations of Lenses
Aberrations of Lenses
Aberrations of Lenses
Aberrations of Lenses
Aberrations of Lenses
Aberrations of Lenses
Aberrations of Lenses
Aberrations of Lenses
Aberrations of Lenses
Aberrations of Lenses
Aberrations of Lenses
Aberrations of Lenses
Aberrations of Lenses
Aberrations of Lenses
Aberrations of Lenses
Aberrations of Lenses
Aberrations of Lenses
Aberrations of Lenses
Aberrations of Lenses
Aberrations of Lenses
Aberrations of Lenses
Aberrations of Lenses
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Aberrations of Lenses

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Aberrations of Lenses - Dr James Beatty

Aberrations of Lenses - Dr James Beatty

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  • 1. Aberrations of Lenses Optics 2005 J.Beatty
  • 2. Introduction  In practice refracting surfaces or systems fall short of theoretical perfection due to aberrations.  The eye converges waves onto retina. The deviation of the converging wavefronts from perfect spheres are called aberations.
  • 3.  We will discuss and describe these aberrations in optical systems, and in the eye, and see how they may be eliminated or reduced.
  • 4. Classification  Chromatic  Monochromatic  For a SPECIFIC wavelength of visible light:  Spherical refractive error(defocus)  Cylindrical (astigmatism)  Diffraction  Spherical aberration  Oblique astigmatism  Coma  Distortion  Curvature of field  Higher order
  • 5. Chromatic Aberration  White light is dispersed into its component wavelengths or colours at an optical surface.  Shorter wavelengths (blue) are deviated more.  Dependent on the optical properties of the material, not the the optical shape.
  • 6.  Corrected in optical systems by:  Achromatic lenses: composed of elements of varying material which neutralizes this dispersion, because the dispersive power is independent of the refractive index.
  • 7.  In the eye:  Chromatic aberration = total dispersion from red to blue of 2.00D.  Emmetropic eye focuses for yellow-green (555nm).  Duochrome test: myopic eye sees red clearly, hypermetropic eye sees green clearly. Very NB clinically as myopes experience eye strain if they are overcorrected. ie. Rather leave them slightly myopic than hypermetropic.  NB: unable to be corrected by refractive surgery (not about shape)
  • 8. Diffraction of Light Interference of light waves with each other at the edge of a wave front.  The result = edges of an image are never sharp.  This limits the amount of magnification that can be attained without losing sharpness.  Clinically: small pupil leads to increased diffraction.
  • 9. Spherical Aberration  Caused by the prismatic effect of the lens => rays passing through the periphery are deviated more.  P = F x D P=prismatic power(D), F = lens power (D), D : decentration (cm).  Marginal focus: point where the peripheral rays converge.  Paraxial focus: point where the the central rays focus.  Circle of least confusion: ¼ the distance b/w the marginal and paraxial focuses.
  • 10.  Corrected in optical systems by:  Occlusion of periphery using stops.  Adjustment of lens form (aplanatic/aspheric).  Use of doublets (principle lens + a weaker opposite power lens of different refractive index).
  • 11.  Corrected in the eye by:  Anterior corneal surface is flatter peripherally.  Iris acts as a stop (optimum pupil 2- 2.5mm).  Lens nucleus has higher refractive index than cortex.  Cones are more sensitive to paraxial than to oblique light (Stiles-Crawford effect = light striking the photoreceptor obliquely is less effective and appears like a shorter wavelength).
  • 12. Astigmatism of Oblique Incidence  Obliquely incident light rays passing through the lens (or looking through the edge of the lens) => toric effect with interval of Sturm, Sturm’s conoid, and circle of least confusion.  Worse with higher power lenses.  Less with meniscus lenses.  NB size of pupil makes no difference.
  • 13.  Corrected in the eye by:  Aplanatic curve of the cornea.  Retina is curved and not flat, therefore the circle of least confusion falls on the retina.  Astigmatic image falls on the peripheral retina, therefore its appreciation is limited.  When prescribing glasses, NB pantoscopic tilt!
  • 14. Coma  Spherical aberration of light from points not on the principle axis.  Rays passing through the periphery are deviated more and have unequal magnification => coma shaped image.  Avoid by limiting rays to the axial area.  Different from spherical aberration, in that the focus is laterally displaced, as apposed to spherical which is longitudinally displaced.  Ocular coma is unimportant in the eye (pupil).
  • 15. Image Distortion  Prismatic effects of lens periphery => uneven magnification.  Concave lens => barrel distortion.  Convex lens => pin-cushion distortion.  Clinically a problem for pts. with high power specs. ie. aphakics.  Pts. can adapt to small amounts of distortion.
  • 16. Curvature of Field  Also know as Petzval surface.  Due to curvature of lens surface and refractive index of lens.  Plane object => curved image.  Cannot be totally corrected under practical conditions.  Compensated for in the eye by curvature of the retina and accommodation.
  • 17. Higher Order Aberrations WAVEFRONT TECHNOLOGY allows us to quantify aberrations.  With this new technology we can identify and quantify third, fourth and fifth order aberrations.  Pattern of aberrations are reproducable.
  • 18. Spherical aberration (Sombrero)
  • 19. Coma
  • 20. Higher order aberrations
  • 21. Post Lasik! (Sombrero hat) (Flipped over sombrero)
  • 22.  PRK and Lasik increase high order aberrations.  The ↑ in aberrations is directly related to ↓ quality of vision, especially under scotopic conditions, low-contrast and glare.  Higher order aberrations cannot be corrected with spherocylinder or std. refractive surgery.

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