The document discusses various types of lens aberrations in optics, which cause image distortion or blurriness due to light not being focused to a single point. It covers specific aberrations such as chromatic aberration, spherical aberration, oblique astigmatism, and distortion types including barrel, pincushion, and mustache distortion, along with their causes and corrective measures. The overall aim is to explain how these aberrations impact image quality and potential solutions to mitigate their effects.

2. LENS ABERRATION
 Aberration in optics refers to a defect in a lens such
that light is not focused to a point, but is spread out over
some region of space, and hence an image formed by
a lens with aberration is blurred or distorted.
 the nature of the distortion depending on the type
of aberration.
 An image-forming optical system with aberration will
produce an image which is not sharp

3. Types of Aberration
Aberration
Chromatic
Longitudinal
Transversal
Monochroma
tic
Spherical
aberration
Positive Negative
Coma
Oblique
Astigmatism
Curvature of
Field
Distortio
n
Pincushion
distortion
Barallel
distortion

4. Chromatic aberration
1- Chromatic aberration is color related
2- It is an effect resulting from dispersion in which there is a
failure of a lens to focus all colors to the same convergence
point.
3- It occurs because lenses have different refractive indices for
different wavelengths of light.
4- It cause an image to have a color fringes along boundaries
that separate dark and bright parts of the image
5- Each color in the optical spectrum cannot be focused at a
single common point.

5. Longitudinal chromatic aberration-
A point light (eg-
White light) forms a
series of point image
along optical axis.
-Shorter
wavelength (blue)
deviate more.
-Longer wavelength
(red) deviate less.

6. Transversal chromatic aberration-
Image of different size
form at the focal
length of lens.
Transverse aberration
occurs when different
wavelengths are
focused at different
positions in the focal
plane
he magnification
and distortion of the
lens also varies with
wavelength

7. Achromatic Lens-
Definition-A Lens would be considered totally without chromatic aberration
is called chromatic aberration.
Principle- All of the wavelength across the visible spectrum focused at one
point when pass through achromatic lens.
Construction of lens- An achromatic lens is constructed from two different
material i.e. a Plus lens of one material is used in
combination with a minus lens of another material
with a different refractive index to counteract the
effect of chromatic aberration.

9. Aberration of lens that occur due to single color of light that
entering the lens system.
These are also called 3rd order aberration.
There are total five types of seidel aberration.
Paraxial equation- F = L’ – L
or
L’ = L + F
This equation is derived from Snell’s law with assumption
that for small angle sin theta = theta

10. Spherical Aberration-
 Caused by increase prismatic effect towards periphery of
lens.
 Object point is on the optical axis of the system
 Peripheral rays deviate more than central ray due to which
peripheral ray focus earlier than central rays.
 Circle of least confusion is ¼ the distance between the
marginal and paraxial focus.
 Two types- Positive and Negative spherical aberration.

11. Positive" spherical aberration means peripheral rays
are bent too much. "Negative" spherical aberration
means peripheral rays are not bent enough

12. 1- By using Stops to occlude peripheral rays.
2- By using Aplantic or Aspheric lenses.
3- By using lens doublet ( Weak lens + a weaker opposite
weaker power lens of different R.I)

13. Correction in eyes-
 By iris which act as stop.
 Anterior surface of cornea is flatter at periphery.
 difference in R.I. from nucleus to cortex of crystalline
lens.

14. Oblique Astigmatism-
 Occurs when rays from off-axis point pass through the
spectacle lenses.
 cause the light to focus as two focal line images known
as Tangential and saggital images.
 The distance between two focus is known as strum's
interval which give the degree of astigmatism.
 When expressed in diopters, this difference is known as
oblique astigmatism error.
 Effect is less with meniscus lenses.
 Effect is worse with high power lenses.

16. Correction in optical system-
 Oblique astigmatism is reduced by finding the
optimum base curve for a given lens power.
Correction in Eyes-
By Aplanatic surface of cornea.
Retina is curved not flat so circle of least confusion fall
on retina
 By prescribing appropriate Pentascopic tilt to lens.

17. Coma
 Occurs when object point is off the axis of the lens.
 Difference in the magnification for rays passing through
different zones of lens.
 Result in the image appear comet or ice cream cone
shaped.
 The point of the cone points towards the optic axis.
 Ocular coma is unimportant in eyes (Pupil).

18. Object plane Image planeLens
Correction-
Avoid by limiting the rays to axial areas

19. Curvature of Field
or
Power error
 Occurs when light entering the peripheral areas of the
lens not focus on the far point sphere.
 Instead it focus on the Petzval surface.
 It arises when wearer looking through the periphery of
lens.
 This aberration cause the spherical component of the
lens to have the effect of being off-power in the periphery
when worn the lenses.
 Also called Petzval curvature of field or Image shell Error.

20. Lens Image shell error
(Petzval surface)
Correction-
1- Cannot be totally corrected under practical condition.
2- In eyes can be corrected by Accommodation and Curvature of
Retina.

21. Distortion
 Occurs because of difference in magnification at different
areas of periphery of lens .
 For + lenses magnification towards the periphery.
 For – lenses magnification towards the periphery.
 It is of three types-
 1- Barrel distortion
 2- Pincushion distortion
 3- Mustache distortion

22. Barrel distortion-
 Image magnification decreases with distance from the optical
axis.
 Straight lines are curved inwards in a shape of a barrel.
 It happens because the field of view of the lens is much
wider than the size of the image sensor and hence it needs to
be “squeezed” to fit.
 As a result, straight lines are visibly curved inwards,
especially towards the extreme edges of the frame.
 Present on most wide angle prime lenses and many zoom
lenses with relatively short focal lengths.
 Correction- Post-processing software such as Lightroom and
Photoshop, can easily fix barrel distortion issues.

23. Pincushion distortion-
 image magnification increases with the distance from
the optical axis
 straight lines are curved outwards from the center.
 The field of view is smaller than the size of the image sensor
and it thus needs to be “stretched” to fit.
 As a result, straight lines appear to be pulled upwards in the
corners in the shape of pincushion.
 This type of distortion is commonly seen on telephoto lenses.
Correction- Post-processing software such as Lightroom and
Photoshop, can easily fix barrel distortion issues.

24. Mustache distortion-
 A mixture of both type of distortion i.e. barrel and
pincushion distortion.
 Also called complex distortion
 It is less common but not rare.
 It starts out as barrel distortion close to the image center
and gradually turns into pincushion distortion towards
the image periphery
 Horizontal lines in the top half of the frame look like
a handlebar mustache.