1. Optical Theory II ABERRATIONS Copyright 2001 -- Ellen Stoner, MALS, ABOM, NCLC

2. 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, 2 nd edition, page 501

3. Classifications of Aberrations Chromatic vs. Monochromatic –Depends on the material of the lens –Requires the beam of light to contain more than one wavelength

4. 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).

5. 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.

6. 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.

7. Lens Aberrations Chromatic Spherical Marginal Astigmatism Coma Curvature of Field Distortion

8. The lens material breaks white light into its component colors Why? Index of refraction varies by wavelength. Chromatic Aberration

9. Longitudinal (axial) The placement of the various focal points on the axis. This is the source of the Abbé value

10. Lateral (magnification) Different image sizes Result in colored ‘ghost’ images

11. 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.

12. 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.

13. Chromatic Aberration Abbé value index  Crown glass581.523  CR-39581.498  PGX571.523  Spectralite471.537  1.6 PGX421.60  Polycarbonate301.586 Brooks & Borish, Systems for Ophthalmic Dispensing 2 nd ed., page 503

14. 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.

15. Lens Aberrations Chromatic Spherical Marginal Astigmatism Coma Curvature of Field Distortion

16. Spherical lens: Peripheral rays have shorter focal length than paraxial rays.

17. 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.

18. Lens Aberrations Chromatic Spherical Marginal Astigmatism Coma Curvature of Field Distortion

19. Spherical lens, narrow beam entering off-axis.

20. 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.

21. 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.

22. 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.

23. Lens Aberrations Chromatic Spherical Marginal Astigmatism Coma Curvature of Field Distortion

24.  Object, way off to the left) Image – cone or comet shaped.

25. 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.

26. Lens Aberrations Chromatic Spherical Marginal Astigmatism Coma Curvature of Field Distortion

27. Plane of focus when Marginal astigmatism is corrected Plane of focus when Curvature of field is corrected

28. 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.

29. 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.

30. Lens Aberrations Chromatic Spherical Marginal Astigmatism Coma Curvature of Field Distortion

31. Distortion – pincushion – high plus lens Object:

32. Distortion Distortion – barrel – high minus lens Object:

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

34. 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.

35. 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)

36. Lens Aberrations Chromatic Sphericalwide beam Marginal Astigmatismnarrow beam Comawide beam Curvature of Fieldnarrow beam Distortion

37. Lens Aberrations Chromatic Sphericalon-axis Marginal Astigmatismoff-axis Comaoff-axis Curvature of Fieldon-axis Distortion

38. Lens Aberrations Chromaticperipheral Sphericalcentral Marginal Astigmatismperipheral (Central when pantoscopic tilt incorrect) Comaperipheral Curvature of Fieldperipheral Distortionperipheral

39. Lens Aberrations In order of importance for lens design: Marginal Astigmatism Curvature of Field..... Distortion Chromatic aberration

40. References Brooks & Borish, System for Ophthalmic Dispensing, 2 nd ed, Butterworth- Heinemann, 1996. Meyer-Arendt, Introduction to Classical and Modern Optics, 4 th ed, Prentice-Hall, 1995.