The document discusses the optical aberrations present in the normal human eye. It describes mechanisms that decrease aberrations, including the iris cutting peripheral rays, the high refractive index of the lens core, and retinal sensitivity patterns. The primary aberrations discussed are diffraction of light forming an airy disc pattern, spherical aberration bringing peripheral rays to different focal points than para-axial rays, coma aberration forming a comma shape, and chromatic aberration focusing different wavelengths to different depths. Decentering of the lens and oblique aberrations from peripheral vision are also mentioned.

2. OVERVIEW
 INTRODUCTION
 MECHANISMS DECREASING Aberrations
 OPTICAL Aberrations OF NORMAL EYE

3. Lapses from perfections – aberrations
Small degree of defects are present for
functional purposes but their presence is
negligible.
Hence, the human eye is by no means optically
perfect.

4. Natural mechanisms to decrease aberration in human eye
 Cutting of peripheral rays by iris
 High refractive index of the core of nucleus of
lens
 Low sensitivity of peripheral retina
 More sensitivity of retina to perpendicular rays
(Stiles-Crawford effect)

5. Physiological
optical defects
in a
normal eye
include

6. Diffraction of light:
• Bending of light rays caused by the edge of an
aperture or the rim of a lens.
• The actual pattern of image
produced by lens is by
the pupil is a series of
concentric bright and dark bands
with a centre bright spot.
• This pattern is termed as airy disc.

7. Spherical aberrations
• Spherical lens refract peripheral rays more than the
para-axial rays
• Which in the case of the
convex lens brings
the more peripheral rays to focus
closer to the lens
Dominant aberrations of the human eye is not spherical
aberration but rather a coma aberration.

8. Coma aberrations
Different areas of the lens form foci in planes other
than the chief focus
This gives a comma effect

9. Chromatic aberrations
The index of refraction of any transparent medium
varies with the wave length of the incident light.
In human eye, the blue light is
focused slightly in front of the red.
Emmetropic eye is
hypermetropic for red rays
myopic for blue and green rays
This forms the basis for BICHROME test
used in subjective refraction.
Image formed

10. Decentring
• The crystalline lens is usually
slightly decentred and tipped
with respect to the cornea and
visual axis of eye.
• The centre of curvature of
cornea is situated abut
0.25mm below the axis of the
lens.
• Deviation is small –

11. Oblique aberrations
• Objects in the peripheral field are seen by
virtue of oblique incident narrow pencil of
rays which are limited by the pupil.
• Because of this the refracted pencil shows
oblique aberration.

12. Thank you…!!!