1) The document discusses various types of optical aberrations including monochromatic and chromatic aberration. 2) For monochromatic aberration, it describes five types: spherical aberration, coma, astigmatism, field curvature and distortion. 3) Chromatic aberration occurs due to unequal refraction of different wavelengths of light, resulting in longitudinal and transverse chromatic aberration. 4) Various techniques are discussed to reduce optical aberrations such as using achromatic lens systems and aspheric surfaces.

1. Aberrations in Optical Aids
Mohammad Arman Bin Aziz
Instructor Optometrist
Institute of Community Ophthalmology

2. Contents of presentation
Definition of aberration
Types of aberration
Reduction of monochromatic aberration.
A discussion of 5 types of monochromatic
aberration.

3. •Monochromatic aberrations:
image is blurred or deformed due to the
approximation error in the paraxial
approximation to the exact solution.
• Chromatic aberrations:
rays corresponding to different wavelengths
travel different paths
Aberrations:
chromatic and monochromatic

4. Monochromatic Aberration
• Monochromatic aberration is due to the
paraxial rays are just an approximation to the
real case.
• For paraxial approximation, we assume but
actually
• The second term are called the third-order
correction to the first order theory (or first-
order corrections to the paraxial theory)
• The third order corrections are
• Spherical aberration
• coma
• astigmatism
• field curvature
• distortion
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5. Monochromatic aberration
• Are image imperfection occurring due to a
form of lens and from monochromatic light.
• In 1850 Seidel described 5 monochromatic
aberration.
• also called as seidel aberration.

7. Seidel aberrations
• Depends on lens diameter, object size and
lens position.
• Independent of wavelength.
• Full correction of one aberration requires
correction of all previous aberrations
• Expressed as 5 terms on coefficients-
• denoted by s1, s2, s3, s4, s5

8. Spherical aberration
• Occurs when a pencil of
light is refracted by a large
aperture system.
• Affects sharpness of
image.
• Occurs because different
zone of aperture lens have
different focal length.

9. • Spherical aberration comes from the spherical surface of a lens
• The further away the rays from the lens center, the bigger the
error is.
• The image is improved if the image plane move closer to the
lens to find optimal spot size

10. Types
• Longitudinal spherical aberration-refers to
separation of paraxial and marginal focus in
diopter along optical axis
• Increases with square of aperture.
• Transverse spherical aberration-measured in
terms of image height differences by two
focuses .
• Increases with cube of aperture

12. Reduction of spherical aberration
stop transmit all rays (only paraxial rays)
Aplantic surface
High powered lenses
Aspheric surfaces

13. Coma
• Occurs when oblique rays are refracted by a
large aperture optical system.
• Affects sharpness of image points.
• Also called as off axial spherical aberration.
• Rarely a problem with spectacle lenses;
limiting effect of pupil .

16. Coma
• Arises from off-axis object points.
• The transverse magnification is a function of ray height
• The resulting pattern is like a comet.

17. • Image resembles a comet or a tear drop.
• Comet shaped patch is formed by a series of
comatic circles .
Reduction of coma-
• controlling aperture size
• Lens form
• Angle of obliquity
• Using aplantic surfaces

18. Oblique astigmatism
• Also called as radial or marginal astigmatism
• Occurs when oblique rays are refracted by a
narrow-aperture system
• Affects both sharpness of image points and
image position.
• Affects the off axial objects point only.

20. • In optical design, the vertical plane is general called the
“tangential plane”
• The “sagittal plane” is the plane at right angle to the tangential
plane and containing the principle ray.
• Astigmatism results in different focusing power to the tangential
and sagittal plane.

21. tangential
sagittal
Astigmatism often arises when focusing with a mirror with an angle

22. • Inability of a lens to form a point image of
an oblique point object.
• Interval of sturms –2 line foci and circle of
least confusion.
• Plane and foci-
• Tangential plane-contains chief ray from
object and optic axis .

24. • Saggital plane –contain chief ray and
perpendicular to tangential plane
• Tangential foci –primary or meridional
foci
• Saggital focus –secondary or
equatorial focus
• Petzvals surface-curved surface along
which image of a planed object lies.

25. • Oblique astigmatism occurs when oblique
power of lens is always weaker than axial
power
• Reduction –use of steeper curvature lens
eg-meniscus lens

26. Curvature of image
• Inability of a lens to form a plane image
of a plane object.
• Image surface is known as petzval
surface.
• Manifest itself as a curved image
surfaces for a flat object surface
• Affects image position.

27. Field Curvature
• a thin lens image a spherical surface onto a spherical surface
• therefore, image is distorted in the image plane
• important in lens design for close objects
Object plane
Image plane

29. Far point sphere
• It is a locus of points conjugate to fovea
as eye rotates .
• Centre is absent only when image
corresponds to far point sphere.

31. Reduction
• Using lens having lower refractive index.
• Slightly overcorrecting myopic eye when
oblique astigmatism is absent .
• Far point of eye –property of eye
• Petzval surfaces –property of correcting lens
• If A is permitted, p can be controlled
• By combining 2 lenses one plus and one
minus.
• Cannot be controlled by altering lens form.

32. Distortion
• Inability of lens to form an image of same
shape as object.
• Occurs when magnification of extended
image varies with distance of corresponding
object from optical axis .
• Affects image shape and lateral position but
not image clarity.

33. • When ratio of image size to object size
has constant value for all object size, no
distortion occurs called as condition of
orthoscopy.
• Pupil of a eye acts as a stop behind a
spectacle lens .

34. • Mainly a problem for lenses of high
power.
• For plus lens ,virtual image suffers a
pinchusion distortion.
• For a minus lens virtual image suffers
barrel distortion.
• Can be reduced by aspheric lenses and
using steeper back surfaces.

35. Distortion
With distortion Corrected
• all points in the object plane are imaged to points in image
plane
• Distortion arises when he magnification of off-axis image is a
function of the distance to the lens center

36. Summary of presentation
Spherical
aberration
coma Oblique
astigmatism
Curvature of
image
distortio
n
Affects
sharpness of
image
Affects
sharpness of
image
Affects both
shapness and
position of
image
Affects
position of
image
Affects
laterality
of image
and
position

37. B. Chromatic Aberration
• When white light is refracted at an optical
interface, dispersed into its component
wavelength or colors
• Shorter the wavelength, more it is deviated
on refraction with series of color images
formed
• Total dispersion from the red to the blue
image being approx 1.5 to 2 D

38. • Emmetropic eye focuses for the yellow
green portion of spectrum, lies approx.
in the middle of the range of retinal
sensitivity
• Thus approx 0.75 to 1.00D of CA lie on
either side of the maximally sharp focus
• For average eye difference between red
and blue foci is 0.5 D
• Caused by material from which lens is
made

39. Types of Chromatic aberration
 Lateral(Transverse CA)
• Defect of an optical system in which size of
image of a point object is extended by a
colored fringe, due to unequal refraction of
different wavelength(dispersion)
• Known as chromatic difference of
magnification

40. • Transverse (lateral) - lens
y
TCAlens = y’C - y’F
TCAlens =
yF
V

41. Chromatic Aberration
• Transverse (lateral) - prism

42. Longitudinal chromatic aberration
• Due to unequal refraction of different
wavelengths(dispersion) which results in an
extended image along the optical axis
• Blue focuses in front of red

43. • Dispersion (dispersive power)
– longitudinal (axial) chromatic aberration of an
optical material:

44. • Longitudinal (axial)
– Bichrome Test (Red/Green Balance)
Lens Retina

45. • Longitudinal (axial)

46. Treatment of chromatic aberration
• Cant be eliminated in an optical element made of
single material
• Use of achromatic lens system, composed of
elements of (lenses)of varying material combined so
that dispersion is neutralized while the overall
refractive power is preserved
• Eg by combining a convex lens of high refractive
power and low dispersive power with a concave lens
of low refractive power but high dispersive power,
aberration can be neutralized

47. Reduction of Chromatic Aberration
• Achromatic lenses
crown
flint
r1 r1’
r2 r2’
r2 = r1’
r1’ = -r1
FTOTAL = F1 + F2
F1 F2
--- +--- = 0
V1 V2

48. n1
n2
R1
R2
R3
• Achromatic doublet (achormat) is often used to compensate
for the chromatic aberration
• the focuses for red and blue is the same if …
Achromatic doublet

49. Reduction of Chromatic Aberration
• Achromatic prisms
crown
flint

50. References
• Optics by Tunnacliffe.
• The Principle of Optics by Hardy and Perrind
• Optics by Fincham Freeman.
• The Art and Science of Optical Design,
R. R. Shannon, Cambridge 1997.