Aberrations of Lenses - Dr James Beatty

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.

4.  We will discuss and describe these
aberrations in optical systems, and
in the eye, and see how they may
be eliminated or reduced.

5. 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

6. 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.

8.  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.

9.  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)

10. 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.

11. 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.

13.  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).

14.  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).

15. 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.

17.  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!

18. 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).

20. 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.

22. 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.

23. 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.

25. Spherical aberration
(Sombrero)

26. Coma

27. Higher order aberrations

28. Post Lasik!
(Sombrero hat)
(Flipped over sombrero)

29.  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.