For OD students by kausar Ali

2. By kausar
ali
student at
OD

3. ABERRATION OF OPTICAL SYSTEM :
 Imperfection on image formation are due to
several mechanism .
 The refracting system of the eye is also
subject to aberrations but there are
correcting mechanism built into the eye itself
.

4. INDEX :
 Chromatic aberration.
 Spherical aberration.
 Oblique aberration .
 coma.
 Image distortion .
 Curvature of field .

5. CHROMATIC ABERRATION:
 When white light is refracted at an optical
interface dispersed into its components
wavelength or color .
 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
.
 A chromatic lens:
special optics design of two mated lens
concave and convex which more precisely
focus the wavelength of light on to the same
plane .

7.  A chromatic lens system are composed of
elements ( lenses ) of varying material
combined so that the dispersion is
neutralized while over all refractive power is
preserved .
 The earliest Achromatic lenses were made
by combining element of flint and crown
glass

8. Single
focal
point
A chromatic lens

9. OCULAR APPLICATION:
 Refraction by the human eye is also subject
to chromatic aberrations the total dispersion
from red to the blue image being
approximately 2 D .
 The emmetropic eye focuses for the yellow –
green (555nm) as this is the peak wave
length of the photopic relative luminosity
curve .
 This wave length this wave length focuses
between the blue and red foci being slighted
near the read .
 Examined by duochrome test

10. DUOCROME TEST:
 The duochrome test can be used to verify the
refraction .
 It is based on the chromatic aberration of the
eye:
 Also based on defocus blur .
 The test is of particular use in the refraction of
myopic patients who experienced eye stain if
they are overcorrected (and those rendered
hypermetropic ) focing them to use their
accommodation for near vision .

11.  Red and green are used because their
wavelength foci straddle the yellow and
green by equal amounts .
 Myopic see red background letter more clear
and hyperopic see green background latter
more clear .
 The test is sensitive to an alteration in
refraction of 0.25 D or less .

12. DIRECTION FOR USE:
 If the near vision is too strong the subject will
be spontaneously see the latters with greater
contrast and blacker on the red background

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

14.  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.

15. 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

16.  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.

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

20. 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.

21.  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.

23. 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.

24. ASPHERIC DOUBLET LENS.

25. 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.

26.  (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.

27. OBLIQUE ASTIGMATISM
 Occurs when rays of light traverse a
spherical lens obliquely… a toric effect is
introduced forming a Sturm’s conoid

29.  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.

30. 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.

31. 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.

33. 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

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