optical aberration is very important for optometrist . eyeball is not optically perfect it shows some optical flaws which reduce resolution of the focused image they are called aberration.

1. Optical aberration
Shubhangi shukla

2. Optical aberration of the eye - eyeball is not optically
perfect it shows some optical flaws which reduce
resolution of the focused images. They are called
aberration.
It is several mechanical operate at different areas
of the eye to eliminate or minimise these aberration.
These are -
● Blocking of peripheral rays of light by the iris
diaphragm.

4. ● Gradient nature of refractive index of the lens
from the central nucleus to the peripheral cortex.
● Aspherical surface of the cornea that is the
peripheral area is flatter and the central area is
more convex.
● Curved surface of the ratina estimates curvature
of field a type of monochromatic aberration.
Aberration may be classified as-

6. Spherical aberration - In a convex spherical lens the
refractive power of the peripheral areas are more
than the central areas so in a convex lens central
parallel rays are brought to a single point focus. It's
peripheral rays come to a focus nearer to the lens
then the point focus. This results in a blurring of
images particularly at the edges. It is termed
positive spherical aberration
A concave spherical lens the peripheral rays come
to a focus after the central ones and it is termed
negative spherical aberration.

7. Spherical aberration is classified under
monochromatic Seidel aberration and it is the only
type which effects parallel rays of light on axis
object so large is dilated pupil contributes to
spherical aberration and visual blur. Spherical
aberration can be neutralized or minimised by either
using a lens in which refractive index varies in a
gradient manner or an aplanatic lens.

9. Oblique/ radial astigmatism - it is axial astigmatism
occurs with toric lens where as spherical exhibit
oblique astigmatism with only oblique rays that is
off axis object. It occurs due to the fact that
spherical lenses show different refractive power in
different meridian only when oblique rays fall upon
them.
Curvature of field - it is closely related to
oblique/radial astigmatism and means that a plane
object gives rise to a curved image. After optical
elimination of spherical aberration coma and radial

10. Astigmatism irrespective of parallel/oblique rays a
point object will form a point image. These point
will be seen to fall on a curved surface (petzal
surface). These phenomena is called curvature of
field.
Due to this curvature of field if an extended image
is projected on a flat surface some of the point will
not be in focus.

12. Distortion - image distortion occurs due to
increased prismatic effect of the periphery of the
lens in difference in lateral magnification at
different point on an object.
Classical ex- distortion is a grid object distorted as
either barrel or pincushion observed in concave
lens and convex lens . Image distortion persists
even often elimination of all other seidal aberration.

14. Wavefront aberration - it is defined as the amount of
difference between an ideal wavefront and the actual
wavefront emanating from an optical system. This
wavefront aberration is measured by the
aberrometry clinical applications of aberrometry
include.
● Corneal refractive surgery.
● Intraocular lenses etc.

15. Coma - it is type of monochromatic aberration that
effect only off axis object oblique rays. Rays passing
through the periphery of the lens are more refracted
than the central rays and come to a focus nearer the
principal axis than the central rays. This results in
unequal magnification of images formed by the rays
passing through different area of the lens. This gives
rises to a composite elongated image simulating a
comet or coma. If the head of the comet point towards
the optical axis is called positive coma while of it point
away from the optical axis it is called negative com.

16. Aplanatic lens can correct both spherical aberration
and coma. A combination of lenses with positive
and negative coma can cancel each other.

17. Chromatic aberration - white light consists of seven
colours. The component colour are refracted
differently at spherical surface and the image formed
have a coloured edges. The short wavelength blue
rays are refracted the most and come to a focus
infront of retina. The red rays with longer
wavelengths are refracted the least and come to a
focus behind the retina. The yellow light rays with
medium wavelength will come to focus in the retina.

18. In human eye visual acuity is surprisingly good
although there should be considerable blurring due to
uncorrected chromatic aberration. These are -
● The xanthophyll pigments present in the macular
area filter out the blue rays the most offender in
chromatic aberration. Luminous efficiency of the
eye is brightest for the yellow coloured rays appear
significantly less bright to cause visual blur.
● The lens even in younger subject filters out uv A
rays.
● Blue rays are filtered out by the lens which turn
yellow in nuclear sclerosis with advancing age.

19. Chromatic aberration is corrected by using achromatic
lens. This is made by combining a convex lens and
crown glass of refractive index 1.523 with concave lens
of flint glass refractive index 1.7 of half diaoptric
strength.

20. Thank you