2. • Optical aberration is an imperfection in the image formation of an
optical system.
• When light from a point source goes through a correctly powered
spectacle lens yet fails to yet fails to create a perfect image, the cause
is lens aberrations.
• Fortunately, the eye possess those defects to so small a degree that,
for functional purposes, their [presence is negligible.
3. Natural mechanisms to decrease aberrations
in normal human eye include:
• Cutting off the peripheral rays by iris,
• High refractive index of the core of nucleus of the lens that that of the
peripheral cortex,
• Low sensitivity of peripheral retina.
• Stiles-Crawford effect- more sensitivity of the retina to perpendicular
rays than the oblique rays.
4. Physiological optical defects in the normal eye
1. Diffraction of light
2. Spherical Aberrations
3. Chromatic Aberrations
4. Oblique Aberration
5. Coma
5. Diffraction of light
• Diffraction is a bending of light caused by the edge of an aperture or
the rim of a lens.
• Even a perfect lens, free from aberrations, will not focus light to a
point due to diffraction.
• The actual pattern of a diffracted image point produced by a lens with
a circular aperture or pupil is a series of concentric bright and dark
rings.
• At the centre of the pattern is a bright spot, known as the Airy disc.
6.
7. Spherical Aberrations
• Spherical aberrations occur because a spherical
lens refracts peripheral rays more strongly than
paraxial rays, which in case of convex lens brings
the more peripheral rays to focus closer to the
lens.
8. Spherical Aberrations
• Factors that contribute in diminishing the spherical aberrations of
human eye:
Peculiar curvature of the cornea, i.e. flatter periphery than the centre.
Peculiar structure of the crystalline lens, wherein the central portions have
greater density and are arrange in layers of greater curvature than the
peripheral portion.
Iris blocks the peripheral rays to enter the eye and thus in ordinary
circumstances, refraction of only paraxial rays of light takes place.
9. Chromatic aberrations
• Caused by dispersion, the variation of
lens’s refractive index with
wavelength.
• In the human eye, which optically acts
as a convex lens, blue light is focused
slightly in front of the red. In other
words, the emmetropic eye is in fact
slightly hypermetropic for the red rays
and myopic for the blue and green
rays. This fact forms the basis of
bichrome test, used in subjective
refraction.
10.
11. • The effect is to a certain extent neutralized by the fact that the eye is
normally focused, so the rays of greatest intensity (the yellow) form
the most sharply defined image, while the colors of longer and
shorter focus form circles of relatively low intensity compared with
this and their images are therefore, neglected.
12.
13. Oblique Aberrations
• The peripheral portion of the lens will form sturm’s conoid, and
therefore in any peripheral oblique axis, two line foci will be formed.
• Reduced by the use of periscopic lenses or meniscus
14. Coma
- Different areas of the lens will form foci in plane
other than the chief focus. The composition image is
not a circle but elongated like comet or coma.
- As with spherical aberration, the limitation of the
rays to the axial areas of the lens can reduce this
effect.
- corrected with parabolic curves and aplanatic lens
design.
