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.
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.
7.
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.
12.
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.
16.
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).
19.
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.
21.
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.
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.
