2. Aberration:
• Aberration is a property of optical systems
such as lenses that causes light to be spread
out over some point.
• Aberration cause the image formed by a lens
to blurred or distorted.
3. Types of aberrations:
There are six major lens aberration:
i. Chromatic aberration
ii. Spherical aberration
iii. Marginal astigmatism
iv. Curvature of field
v. Coma
vi. Distortion
4. Chromatic aberration
• Chromatic aberration is a defect in the lens in which the
various colors of the spectrum are not brought to the same
focus.
• Blue light is refracted more than the red light when it
passes through a lens the result is out of focus.
• The higher the power of the lens the greater is the
chromatic aberration.
• Chromatic aberration depends upon the material of the
lens. Since the lens materials have a different refractive
index for each wavelength
• The lens will have a different focal length for each
wavelength. The refractive index is larger for blue than the
red wavelength so focal length is less for blue than the red.
6. Correction:
• The easiest solution to minimize chromatic aberration
is to change the lens material to higher abbe value.
• Careful placement of optical centre with monocular
pupillary distance and its height in a small frame may
reduce the chromatic aberration
• Reducing the vertex distance may also result in
minimizing the effect of chromatic aberration
• Anti-reflection coating with consumer education may
also be tired to minimize the effect of chromatic
aberration
• The best solution is achromatic lens system
8. Ocular chromatic aberration:
Refraction by the human eye is also subject to
chromatic aberrations. As a result the focusing
power of the eye is different for different
wavelength of light. The blue wavelength
focuses before the retina and red beyond the
retina. The cause of chromatic aberration are
dispersion in the cornea, aqueous , crystalline
lens and vitreous humour.
9. Spherical aberration
• Spherical aberration is an axial and wide beam
aberration
• The light rays from the peripheral edge of the
lens are refracted to a greater degree then the
light rays passing through the centre of the
lens.
• Peripheral rays bend more than the paraxial
rays this creates a slight blurring of the image
that is minimized by the size of the lens.
11. Correction
• Spherical aberration may be reduced by
occluding the periphery of the lens such that
only the paraxial zone is used.
• Lens form may also be adjusted to reduce
spherical aberration.aplanatic surface where
periphery curves is less than the central
curvature may be used.
13. Ocular Spherical Aberration
• Refractive power of the ocular structure is
greater for peripheral rays than paraxial
rays.The effect of spherical aberration on
retinal image is a symmetrical blur like defocus
which is reduced by the size of pupil.
14. Marginal Astigmatism
• Marginal astigmatism or oblique astigmatism
aberration is a small angle aberration
• When a narrow beam of light enters obliquely
to lens axis of spherical lens the refracted ray
become astigmatic
• The emergent ray instead of uniting in a single
image point form two foci at right angles to
one another with a disk of least confusion
16. Correction
• It may be reduced by the use of aspheric
surface or by a suitable choice of lens
bending.
• Proper use of pantoscopic tilt with optical
centre height may help reducing marginal
astigmatism
17. Ocular Astigmatic Aberration
Oblique rays passing through the pupil of the
eyes create astigmatic aberrations in human
eye. The visual effect is not more than
reduction of contrast.
18. Coma
• Coma is a wide beam aberration and is similar
to spherical aberration
• Oblique rays passing through the periphery of
the lens are deviated more than the central
rays and come to focus near the principal axis
• The result is unequal magnification of the
image formed by the different zones of the
lens. The composite image is not circular, but
elongated like a comet and coma.
20. Correction
• The effect of cometic aberration can be
minimized by parabolic curve.
• Aspheric lens design helps reduce coma in
high plus power.
21. Ocular coma
In human eyes coma gives a comet like image
because of small decentration of the cornea
and lens which result in different
magnification in the different parts of the
pupil.
22. Curvature of field
• Curvature of the field is a phenomenon which
causes the image formation of a plane to
become curved like.
• The effect is largely depend on refractive index
of the lens material and curvature of the lens
surface.
24. Correction
• Curvature of field affects peripheral vision.
Curvature of field is minimized with corrected
curve design base curvatures. As a rule if
marginal aberration is corrected, the effect of
curvature of field is also reduced. Good
correction of astigmatism results in small
curvature of field.
25. Ocular curvature of field
• In the human eye the curvature of the retina
compensates for the effect of curvature of
field. So this is an advantageous aberration in
the human eye.
26. Distortion
• Distortion is another aberration of thick lenses. In
distortion the object is sharply imaged but does not
retain its shape.
• There are 2 types of distortion
1. Barrel distortion: it is produced in minus power lens
where the rays in the centre are more than the rays in
the off-axis. This is due to the minification of corner
of the square grid.
2. Pincushion distortion: it is produced in plus lens
where the central rays are less magnified. This is due
to the magnification of the corner of the square grid