Aberrations are defects in an optical system that prevent light from being brought to a single focus. There are two main types: chromatic aberration, caused by the wavelength-dependent refractive index of materials, and monochromatic aberration, caused by flaws in the lens design. Five primary monochromatic aberrations were described by Seidel: spherical aberration, coma, astigmatism, field curvature, and distortion. Spherical aberration occurs when peripheral rays converge at a different point than paraxial rays, degrading image focus. Coma and astigmatism affect image sharpness for off-axis points. Field curvature results in a curved image plane rather than a flat one. Distortion al

1. Aberration
Presented by:
Ashi Lakher
B. Optometry
Final year

2. Introduction
• In an ideal optical system, all rays of light from a point in the object
plane would converge to the same point in the image plane, forming
a clear image. The influences which cause different rays to converge
to different points are called aberrations.
• A defect in lens design which causes light to spread out instead
of focusing to form a sharp image.

3. Types of Aberration

4. Chromatic Aberration:
• Chromatic aberration is due to fact that the refractive index of a
material varies slightly with the wavelength.
• CHROMATIC ABERRATION:
1. Longitudinal (axial) chromatic aberration
2. Transverse (lateral) chromatic aberration.

6. Clinical implication :

7. PARAXIAL ASSUMPTION
• For small angle , i.e Paraxial rays makes
SinƟ = Ɵ, (Ɵ= radian)
• But as the angle of incidence increases, better estimation of sinƟ given by
following expansion:
• When 3rd order aberration {Ɵ-(Ɵ3/3!)} is used , image formation differs
from what is predicted by paraxial equation

8. Monochromatic aberration:
• In 1850’s Ludwig von Seidel described 5 monochromatic aberrations which
affect the image when the object is far enough off axis or the area of the
lens used is far enough from the axis.
• Monochromatic aberrations, a/k/a Seidel aberrations
Spherical aberration
Coma
Oblique astigmatism
Curvature of image
Distortion

9. Spherical aberration:
• It is due to prismatic effect of lens.
• The collimated rays entering the lens at different distances from the
optical axis will converge to different points, causing an overall loss of
focus. Like many optical aberrations, the blur effect increases towards
the edge of the lens.
• Light striking periphery of lens deviates more while paraxial light deviates
least, that difference of focal point of peripheral and paraxial rays is
Spherical aberration.

10. • When peripheral rays deviates more is called Positive and when least is
called Negative Spherical aberration.

12. More SA
Less SA

13. Clinical implication:
• Iris prevent SA by blocking peripheral rays.
• Unaccommodated eyes has positive SA.
As eye accommodates – Amount of positive SA decreases
• Night myopia(In dim light pupil dilates ,peripheral rays reaches retina
and eye is myopic )

14. COMA:
• When the point source is moved off axis , the illumination pattern on
the screen at the paraxial image position frequently changes to a
comet shaped pattern with head and tail. This seidel aberration is
coma.
• It affects the sharpness of image points.

15. POSITIVE AND NEGATIVE COMA:

16. Radial Astigmatism :
• For an off axis point source ,RA results in a centered spherical system
producing a conoid of Sturm with two perpendicular line image - One
associated with the tangential meridian and one associated with
sagittal meridian.
• The line image form by tangential meridian is horizontal line and
image form by sagittal meridian is vertical line.

18. • tangential image surface is curved more than sagittal image surface.

19. Petzval surface
• When lens form an image of plane object, the image lie along a
curved surface which is known as petzval surface.
• Tangential & Sagittal foci and
Petzval surface

20. Tscherning ellipse
Teacup and saucer diagram formed by
extended object subjected to oblique
astigmatism.

21. Clinical implication:
• Even when oblique astigmatism is minimized through proper
selection of the front surface power, it can still be problematic when
the lens is tilted with respect to the eye.
• example, wrap-around sunglasses .
The effective lens power induced by
face form can be calculated by formula:

22. • Pantoscopic tilt
Pantoscopic tilt increases a minus lens’s minus power. It is for this
reason that undercorrected myopic patients sometimes intentionally
tilt their spectacles to improve distance vision.

23. CURVATURE OF IMAGE :
• In the absence of other aberration , plan object forms the curve
image which is known as curvature of image.

24. • This is because all the points on the extended object are not at the
same distance from spherical converging lens
• Principally problem for optical instrument (especially for camera )
because camera use plan image film for image capturing.
• Less problem with eye because retina is curved.

25. Distortion :
• Distortion is due to fact that the lateral magnification for off-axis
points depend on the exact image and object distance along chief
rays.
• It does not cause any blur or loss of resolution.

26. Symmetrical:
Pincushion distortion
• It occurs if image size to object size ratio increases with an increase in
object size.
• Plus lens cause pin cushion distortion.
Barrel distortion
• It occurs if image size to object size ratio decreases with an increase
in object size.
• Minus lens cause barrel distortion.

27. Asymmetrical:
• when formed by non-centered optical system(prism produced
asymmetrical distortion).