Ophthalmic lens aberration

Ophthalmic lens aberration

1. Ophthalmic Lens
Aberrations
OPTOM FASLU MUHAMMED

2. •In actual world, the eyes turn behind the lens to view
through off-axis visual points and it is then that the
form assumes importance.

3. Lens Aberrations
•There are six major lens aberrations that work against
obtaining a perfect image through the periphery of
the lens.

4. •CHROMATIC ABERRATION
•SPHERICAL ABERRATION
•MARGINAL ASTIGMATISM
•CURVATURE OF FIELD
•COMA
•DISTORTION

5. CHROMATIC ABERRATION

6. •Chromatic aberration is a defect in the lens in which
the various colours 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
image.

7. •The wearer complains of peripheral colour fringes
around the object which is more pronounced off-axis.
•The higher the power of the lens—the greater is the
chromatic aberration.

8. •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.

9. Importance
•Since chromatic aberration occurs because the
refractive index of the lens material varies with the
wavelength of the incident light.
• It gives rise to what is called the Abbe value of the
lens material which is denoted by V value.

10. •Higher Abbe value implies low chromatic aberrations
and vice versa.
•So polycarbonate lens with Abbe value of 30 causes
more chromatic aberration than CR39 lens with Abbe
value of 58.

11. Correction
1.The easiest solution to minimize chromatic
aberration is to change the lens material to higher
abbe value.
2. Careful placement of optical centre with
monocular pupillary distance and its height in a
small frame may reduce the chromatic aberration.

12. 3. Reducing the vertex distance may also result in
minimizing the effect of chromatic aberration.
4. Anti-reflection coating with consumer education
may also be tried to minimize the effect of chromatic
aberration.

13. 5. The best solution is the Achromatic
lens system.

14. Achromatic lens
•The achromatic lens uses two different lens materials
– one has a regular focal length and the other corrects
the dispersion of the first lens.
•For this purpose one lens is made of crown glass, i.e.
low dispersion, while the other is made of flint glass,
i.e. high dispersion.

15. Ocular Chromatic Aberration

16. 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 than the light rays
passing through the center of the lens.

17. •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.

18. MARGINAL ASTIGMATISM

19. 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 a spherical lens, the refracted rays become
astigmatic.
•The emerging rays, instead of uniting in a single
image point, form two foci at right angles to one –
another with a disk of least confusion.

20. Correction
•It is much worse in bi-convex and bi-concave lens
than meniscus lens form.
•It may be reduced by the use of an aspheric surface or
by a suitable choice of lens bending.
•Proper use of pantoscopic tilt with optical centre
height may help reducing marginal astigmatism.

21. COMA

22. •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.

23. Correction
•The effect of cometic aberration can be minimized by
using parabolic curves.
•Aspheric lens design helps reduce coma in high plus
power.

24. CURVATURE OF FIELD

25. DISTORTION
•Distortion is another aberration of thick
lenses. In distortion the object is sharply
imaged but does not retain its shape.
Barrel Distortion
Pincushion Distortion

26. Barrel Distortion
•Barrel distortion is produced in minus power lens
where the rays in the centre are more magnified than
the rays farther off-axis.
•This is due to minification of corners of a square grid
more from minus lens.

27. Pincushion Distortion
•Pincushion effect is produced in plus lens where the
central rays are less magnified. This is due the
magnification of corners of a square object more
from plus lens

29. Thank u……