This document discusses different types of optical aberrations, including chromatic aberration and monochromatic aberrations. Chromatic aberration occurs due to the refractive index varying with wavelength, while monochromatic aberrations consider a single wavelength. The five types of monochromatic aberrations described are spherical aberration, coma, oblique astigmatism, Petzval field curvature, and distortion. Causes and methods to minimize each aberration type are provided.

1. Aberrations
By:
Saikarthik.s
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INTRODUCTION
 Aberration is derived from a Latin word
“aberrationem” meaning "a wandering, straying"
 Aberrations in optics means faults in an image formed by
an optical system
 There are two types of aberration:
 Chromatic aberration.
 Monochromatic aberrations.
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 Chromatic aberration
 it is re-laid on the principle of prism dispersion
of a white coloured light
 It occurs because the refractive index of a
material varies with the wavelength of the
incident light
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 Monochromatic aberrations
 This is based on the consideration of a single
coloured light of a particular wavelength
 It is a fault in an image due to imperfect focusing
of light rays by an optical system
 There are five types of monochromatic
aberrations they are:
(i) Spherical aberration.
(ii) Coma.
(iii) Oblique astigmatism.
(iv) Petzval field curvature.
(v) Distortion. 4

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Spherical aberrations
 In the case of spherical aberrations with parallel light
rays from the object , the marginal light rays of the
object is focused before the point of focus of paraxial
light rays
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 Spherical aberration can be minimised by:
1.Limiting the aperture of the lens so that only paraxial
rays are refracted.
2.Adjusting the form of the lens (best form lens concept
by adjusting the suitable base curves)
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COMA
 Coma is the second Seidel aberration.
 It can be thought of as off-axis spherical
aberration and is visible when image forming
light rays are oblique
 There are three conditions in coma
 Positive coma
 Negative coma
 No coma
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 coma can be reduced in three ways
1.Reducing the aperture of the lens so that only
paraxial rays are refracted
2.Adjusting the shape of the lens
3.Correct positioning of a stop in front of a lens
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THE OBLIQUE ABERRATIONS
 This type of aberration is caused due to the
formation of two focus by the oblique light rays
from an object
 The image formation in an oblique astigmatism
is traced with a image shell diagram
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Image shell diagram
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 Oblique astigmatism for a lens or lens system
can be minimised by:
1.Correct choice of lens form.
2.Appropriate positioning of an aperture.
3.Using an aspherical surface.
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Petzval field curvature
 If a surface did not exhibit oblique astigmatism,
the two line images would collapse to a point,
which would fall on a single image shell known
as the Petzval image surface
 The petzval surface thus formed must coincide
with the curvature of the retina
 It could be minimized with best form lenses
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DISTORTION
 The final Seidel aberration is distortion
 Even if all the other four monochromatic
aberrations are absent a lens or lens system may
still suffer from distortion
 Mostly seen in high power plus and minus lenses
of the aphakic patients
 There are two types of distortion, pincushion
distortion and barrel distortion.
 Distortion occurs because the linear
magnification of the image varies over the image
field
 Lenses that are free from distortion are said to be
“Orthoscopic” 15

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 In high power plus lenses , pincushion distortion
is observed due to over magnification effect the
field of view is confined with reduced marginal
vision
 In high minuses , barrel distortion is observed
due to over minification providing more curved
marginal vision
 It could be minimized by adjusting the position of
aperture
normal plane barrel distortion pincushion distortion
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Does a normal emmetropic eye avail aberrations ???
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