This document discusses various types of optical aberrations including chromatic aberration, spherical aberration, oblique astigmatism, and coma. It explains how each aberration occurs in optical systems and lenses. It also describes how the human eye corrects for these aberrations through mechanisms like the aplanatic corneal surface and retinal shape. Chromatic aberration is corrected in optical lenses using achromatic lenses made of materials with different dispersive properties. Spherical aberration and coma are reduced by limiting the optical zone or using aspheric surfaces.

1. Aberrations of
Optical Systems
Othman Al-Abbadi, M.D

2. • Imperfections of image formation are due to several
mechanisms
• The refracting system of the eye is also subject to aberrations,
but there are correcting mechanisms built into the eye itself.

3. Index
• Chromatic aberration
• Spherical aberration
• Oblique astigmatism
• Coma
• Image distortion
• Curvature of field

4. Chromatic Aberration
• When white light is refracted at an optical interface, it is
dispersed into its component wavelengths or colors .
• The shorter the wavelength of the light, the more it is
deviated on refraction.
• Thus a series of colored images are formed when white light
is incident upon a spherical lens

6. Correction of Chromatic
Aberration
• The dispersive power of a material is independent of its
refractive index.
Thus, there are materials of high dispersive power but low
refractive index, and vice versa.

10. Achromatic Lens
• Special optics design of two mated lens – concave and convex
– which more precisely focus the wavelengths of light onto
the same plane.
• Achromatic lens systems are composed of elements (lenses)
of varying material combined so that the dispersion is
neutralized while the overall refractive power is preserved

11. • The earliest achromatic lenses were made by combining
elements of flint and crown glass.

13. Ocular application
• Refraction by the human eye is also subject to
chromatic aberration, the total dispersion from
the red to the blue image being approximately
2-D.
• The emmetropic eye focuses for the yellow–
green (555 nm) as this is the peak wavelength of
the photopic relative luminosity curve.
• This wavelength focus lies between the blue and
red foci, being slightly nearer to the red
• Examined by duochrome test

14. photopic luminosity curve

15. DUOCHROME TEST
• The Duochrome test can be used to verify the
near addition.
• It is based on the chromatic aberration of the
eye.
• The test is of particular use in the refraction of
myopic patients, who experience eye strain if
they are overcorrected (and thus rendered
hypermetropic), forcing them to use their
accommodation for near vision.

16. • Red & green are used because their wavelength
foci straddle the yellow-green by equal amounts
~0.4D
• Myopics see red letters more clearly, & vice
versa.
• The test is sensitive to an alteration in refraction
of 0.25 D or less.

18. Directions for use :
• If the near vision correction is too strong, the subject will
spontaneously see the letters with greater contrast and
blacker on the red background.

19. If the letters are seen clearer on the green
background, it means that the near
correction is too weak.

20. • Color blindness doesn’t invalidates the test due to its
dependence on the position of the image with respect
to the retina NOT on color discrimination.
• A color-blind should be asked which side’s letters
appears clearer.
• The eye with overactive accommodation may still
require too much minus sphere in order to balance
the red and green. Cycloplegia may be necessary.

21. Spherical Aberration
• It was seen that the prismatic effect of a spherical lens is least
in the paraxial zone and increases towards the periphery of
the lens.
• Thus, rays passing through the periphery of the lens are
deviated more than those passing through the paraxial zone
of the lens

22. • In other words, the parallel light rays of incoming light do
not converge at the same point after passing through the
lens. Because of this, Spherical Aberration can affect
resolution and clarity, making it hard to obtain sharp
images.
• Results in out-of-focus image.

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

26. Correction of Spherical
Aberration
• Spherical aberration may be reduced by occluding the
periphery of the lens by the use of 'stops' so that only
the paraxial zone is used.

27. • To achieve the best results, spherical surfaces must be
abandoned and the lenses ground with aplanatic surfaces;
that is, the peripheral curvature is less than the central
curvature .
• Aspherical lenses are lenses with complex curved surfaces,
such as where the radius of curvature changes according to
distance from the optical axis.

28. aspheric doublet lens
• Another technique of reducing spherical aberration is to
employ a doublet. This consists of a principal lens and a
somewhat weaker lens of different refractive index cemented
together .
• The weaker lens must be of opposite power, and because it
too has spherical aberration, it will reduce the power of the
periphery of the principal lens more than the central zone.
• Usually, such doublets are designed to be both aspheric and
achromatic.

29. aspheric doublet lens.

30. Ocular application
• The effect of spherical aberration in the human eye is
reduced by several factors:
• (1) The anterior corneal surface is flatter peripherally
than at its centre, and therefore acts as an aplanatic
surface.
• (2) The nucleus of the lens of the eye has a higher
refractive index than the lens cortex… Thus the axial
zone of the lens has greater refractive power than the
periphery.

31. • (3) The iris acts as a stop to reduce spherical aberration.
The impairment of visual acuity that occurs when the
pupil is dilated is almost entirely due to spherical
aberration (Optimum pupil size is 2–2.5 mm.)
• (4) Retinal cones are much more sensitive to light which
enters the eye paraxially than to light which enters
obliquely through the peripheral cornea (Stiles–Crawford
effect).
• This directional sensitivity of the cone photoreceptors
limits the visual effects of the residual spherical
aberration in the eye.

32. Oblique astigmatism
• Occurs when rays of light traverse a spherical lens obliquely…
a toric effect is introduced forming a Sturm’s conoid

34. • Occurs with spectacle lenses when the light of sight is NOT
parallel with the principal axis of the lens.
• Worse with higher power lenses.
• Less with meniscus (convex-concave) lenses.
• NB size of pupil makes no difference
• Can be corrected by Pantoscopic tilt of the glasses due to the
fact that adults spend most of their time looking slightly
downward from the primary position.

35. Ocular application
• Occurs in the eye but its visual effect is minimal… Due to:
1.Aplanatic surface of the cornea reduces oblique astigmatism
as well as spherical aberration
2.Retina is a spherical surface ; the circle of least confusion of
the Sturm’s conoid formed by oblique astigmatism falls on the
retina.
3.Astigmatic image falls on peripheral retina which has poor
resolving power compared to the macula; visual appreciation
of astigmatic image is limited.

36. Coma
• Spherical aberration applied to light coming from points NOT
lying on the principal axis.
• Rays passing through the periphery of the lens are deviated
more than central rays & come to a focus nearer the principal
axis.
• Results in unequal magnification of the image formed by
different zones of the lens.
• Differs from spherical aberration in that the image formed is
laterally displaced.

37. Ocular application
• May be avoided by limiting to the axial area of the lens.
• Not of clinical significance due to the same reasons for oblique
astigmatism… which are:
1.Aplanatic surface of the cornea
2.Retina is a spherical surface
3.Coma image falls on peripheral retina which has poor
resolving power compared to the macula; visual appreciation
of astigmatic image is limited

41. Wave front technology