This document discusses various types of optical aberrations including monochromatic and chromatic aberrations. Monochromatic aberrations include spherical aberration, which causes rays passing through the periphery of a lens to focus differently than central rays. Coma causes off-axis object points to have disparate focal lengths. Chromatic aberration results in different wavelengths of light focusing at different distances, with blue light focusing closer than red light. The human eye exhibits these aberrations but various anatomical features help reduce their effects on vision.

1. Aberrations of the Optical System
including the Eye
By: Fajri mohammed (MD )
ophthalmology R. (R1 )
JUDO January , 2022
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2. out line
• Introduction
• Monochromatic aberration
Spherical aberrations
Oblique Astigmatism
Coma
 Distortion
Curvature of Field
• Chromatic Aberrations
• Summary
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3. Introduction
• Aberrations defined as the d/c in optical path length (OPL) between any ray passing
through a point in the pupillary plane and the chief ray passing through the pupil center.
• Its deviations from stigmatic imaging.
• It is an imperfection of image formation which leads to blurring and distortion of the
image formed by an optical system.
• Divided into;
• Monochromatic
• Chromatic aberration
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4. Monochromatic aberrations
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• Are independent of chromaticity of the light &
Caused by the geometry of the lens
Spherical aberration
Marginal Astigmatism
Coma
Curvature of Field
Distortion

5. Cont.…
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6. cont.…
Spherical Aberration
• By high powered optical system
• Rays passing through the periphery of
the lens are deviated more than those
passing through the paraxial zone
• The further away a ray is from the axis,
the more closer its focus to the lens
will be.
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7. Cont.…
• Marginal focus: point where peripheral rays converge
• Paraxial focus: point where central rays focus
• Circle of least confusion: ¼ of distance b/n marginal & paraxial focus/ at
diopterical center of focused image of two meridian.
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8. Cont…
• Spherical aberration has 2 effects:
Image quality (VA) decreases --- the focus is not stigmatic
Image location is changed from predicted position
• The best focus is at where the rays are confined to the smallest area
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9. Cont.…
• In the human eye, spherical aberration shifts the focus anteriorly
• Making the patient slightly more myopic than would be expected from vergence
calculations
• Increase with power of lens and width of beam
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10. Correction of Spherical Aberration
• Use of diaphragm
Occluding the periphery of the lens using ‘stops’
• Adjusting lens form
 Plano-convex is better than biconvex
• Making the peripheral curvature less than the central
curvature(aplanatic (aspheric) surfaces
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11. cont.…
• Use of doublet
Principal lens plus a somewhat weaker lens of different refractive index
cemented together
The weaker lens must be of opposite power
This reduce the spherical aberration (aspheric) and possibly the chromatic
aberration (achromatic)
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12. Ocular Spherical Aberration
• Spherical aberration accounts for some cases of fluctuating vision following
keratorefractive surgery
• Radial keratotomy makes the cornea more spherical increasing spherical
aberration.
• Laser in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK)
can make the central cornea flatter than the peripheral cornea
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13. cont.…
 Night myopia
• Spherical aberration exacerbates myopia in low light due to:
Pupil dilation with decreased light
Focus shifts anteriorly
The amount of myopic shift is about 0.50 D
• The effect of spherical aberration increases as the fourth power of the pupil
diameter
• Small change in pupil size can cause a significant change in refraction
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14. cont.…
Effect of spherical aberration in the human eye is reduced by:
• Anterior corneal surface is flatter peripherally than at its center so it acts as an
aplanatic surface
• The nucleus of the lens has a higher refractive index than the cortex.
Thus the axial zone of the lens has greater refractive power than the
periphery .
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15. Cont…
• The iris acts as a stop to reduce spherical aberration.
 Impairment of VA when the pupil is dilated is almost entirely due to spherical
aberration.
 Optimum pupil size is 2–2.5 mm.
• Retinal cones are much more sensitive to light entering the eye par axially than
obliquely through the peripheral cornea (Stiles–Crawford effect).
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16. Oblique Astigmatism
• Occurs when rays of light traverse a spherical
lens obliquely.
• This introduces a toric effect and emerging rays
will forms a Sturm's conoid with two line foci.
• Occurs in spectacle lenses when the line of sight
is not parallel with the principal axis of the lens.
• Is un avoidable in the case of the near portion of
a multifocal lens
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Oblique astigmatism. FH and FV represent
the horizontal and vertical line foci of a
Sturm's conoid

17. Cont.…
• It reduce VA in patients with restricted eye movement, who
adopt a compensatory head posture and
 look obliquely through peripheral portions of their spectacle lenses.
• The higher the spectacle lens power, the greater the unwanted cylindrical power
induced by the aberration
• It is considerably affected by the form of the lens used
Much worse in biconcave & biconvex lenses than in meniscus lenses
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18. How to overcome oblique astigmatism
• Using Best form lenses
Are optimum form of single lenses for reducing both spherical and
oblique aberrations which is usually in meniscus form.
• Pantoscopic tilt
 Results when the frame is rotated with respect to the face’s
vertical frontal plane.
Is the tilt of lower border of spectacle lenses towards cheek
 The axis of the induced cylinder is 180 degrees.
It reduces the obliquity of the reading portion of multifocal
lenses
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19. Cont.…
• Pantoscopic tilt bring near zone closer to the eye --- increase field of view through
near zone of lens
• It may cause intolerance in high power spectacle wearers if new frame having
different angle of tilt are dispensed
• pantoscopic tilt increases a minus lens’s minus power.
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Induced cylinder (axis 180) = F (tan2θ)

20. Eg .What is the effective power of a -8.00 DS polycarbonate lens that is
mounted in a frame that has a pantoscopic tilt of 15 degrees?
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21. Ocular Oblique Astigmatism
• Occurs in the human eye but its visual effect is minimal.
• The factors which reduce ocular oblique astigmatism are:
 The aplanatic curvature of the cornea reduces oblique astigmatism as well as
spherical aberration.
 The astigmatic image falls on peripheral retina which has relatively poor resolving
power compared with the retina at the macula so Visual appreciation of the
astigmatic image is therefore limited.
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22. Cont.…
• The retina is not a plane surface, but a spherical surface.
The radius of curvature of the retina in emmetropic eye means that the circle of least
confusion of the Sturm's conoid formed by oblique astigmatism falls on the retina
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23. Coma
• Coma is a disparity in focal length for rays from a single off-axis object point that are
refracted at different distances from the center of the lens.
• Occurs in large aperture optical system
• Oblique Rays passing through the periphery are deviated more than the central rays ---
that come to a focus nearer the principal axis
• Results in unequal magnification of the image formed by different zones of the lens
• The composite image is elongated like a coma or comet
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24. Cont.….
Correction of Coma Aberration
• Ways of correction:
Limiting rays to the axial areas of lens
Using principal axis of the lens rather than a
subsidiary axis
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25. Distortion
• It is a disparity in transverse magnification for objects at different distances from the
optic axis.
• Is inability of lens to form image of the same shape as an object
is due to increased prismatic effect at the periphery of the lens --- uneven
magnification of the object
• Image is in focus clearly; only shape is distorted
• Create inconvenience in users of high-power spectacle lenses (aphakic)
• Can be reduced with aspheric lenses
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26. Types of distortion
• Barrel distortion
 Occurs in minus lenses
 Rays in the centre are more
magnified than the further
• Pin-cushion’ distortion
 Produced in plus lenses
 Rays in the centre are less magnified ---
magnification of corners
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27. Curvature of Field
• Also called petzval surface
• Indicates a plane object gives rise to a curved
image
• Affects image position
• Occurs even with eliminated spherical aberration,
oblique astigmatism & coma .
• This effect is dependent upon:
The refractive index of the lens material
The curvature of the lens surfaces
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28. Cont…
• In the eye the curvature of the retina compensates for aberration of curvature of
field.
• ”Since off-axis rays that pass through the center of a lens can cause curvature of field,
this aberration is clinically important in the design of spectacle lenses.
• It can be minimized by the proper selection of the lens front surface power.
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29. Measurement of the Eye’s Monochromatic Aberrations
• The eye’s monochromatic aberrations can be measured using devices called
aberrometers.
• Perhaps most widely discussed is the Hartmann-Shack aberrometer,
• An advantage of this procedure is it allows the quantification of aberrations in non
spherical optical systems (such as the eye).
• To measure aberrations in any optical system, we compare the OPL of a light ray
passing through any point (x,y) present at the plane of the pupil with the chief ray
passing though the center of the pupil (0,0).
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30. Cont..
• The manner in which the pattern deviates
from a regular grid reveals the nature of
the eye’s aberrations and is quantified as
Zernike polynomials (i.e., second order,
third order, etc.).
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31. Chromatic Aberration
• When white light is refracted at an optical interface, it dispersed
• The shorter the light wavelength, the more deviated on refraction
• Thus a series of coloured images are formed when white light is incident upon a
spherical lens
• This separation of white light into its component elements by a prism (or other optical element) is
referred to as chromatic dispersion.
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32. Cont…
• Dispersive power of a material is independent of its refractive index
• Dependent on optical properties of material, not on optical shape
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33. Cont…
Dispersive Power and Constringence
• Lens manufacturers use the Abbe number to specify the chromatic aberration of their
products 33

34. Cont…
• Two types of Chromatic Aberration
Lateral (Transverse) Chromatic Aberration
 Longitudinal Chromatic Aberration
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35. Lateral (Transverse) Chromatic Aberration
• We can define it as the difference in prismatic power for wavelengths of 486 and 656
nm
• The chromatic aberration produced by a prism
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36. Cont.…
• Do lenses manifest lateral chromatic aberration?
• Lateral chromatic aberration is an important consideration when prescribing lens
materials with low Abbe numbers, such as polycarbonate.
• As the patient looks through more peripheral regions of the lens, the prismatic
power increases, resulting in greater lateral chromatic aberration.
• This may be seen by the patient as colored fringes.
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37. Longitudinal Chromatic Aberration
• The lens’s longitudinal chromatic aberration is defined as The difference in dioptric
power for wavelengths of 486 and 656 nm. Stated as an equation,
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38. Ocular Chromatic Aberration
• Refraction by human eye is also subjected to chromatic aberration
• Chromatic aberration of the eye focuses blue light in front of red light
• The total dispersion from red to blue image is about 0.5- 2.00D.
• The emmetropic eye focuses for the yellow–green (555 nm)
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39. Duochrome test
• Also called red-green test or bichrome test
• The test consists of two ranks of black Snellen letters
• With illumination, the patient can easily tell which side appear clearer
Myopic eye sees the red letters more clearly while
Hypermetropic eye sees green letters more distinctly
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40. Cont.…
• The test is sensitive to an alteration in refraction of 0.25 D or less
If the letters of the green side are clearer +0.25 D sphere is added
If the letters on the red side are clearer -0.25 D sphere is added
• When the image is clearly focused in white light, the eye is
 0.25 D myopic for the green symbols
 0.25 D hyperopic for the red symbols
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41. Cont.…
• With optimal correction, the letters on the either sides will be equally clear.
• The test is used to verify the spherical endpoint
• Particularly useful in the refraction of myopic patients, who experience eye strain
if they are overcorrected.
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42. Cont…
• Colour blindness does not invalidate the test because:
It depends on the position of the image with respect to the retina
Is based on chromatic aberration, not on colour discrimination
Pt should be asked whether the upper or lower rank of letters clearer.
• The test is not used for patients with VA < 6/9
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43. Correction of Chromatic Aberration
Achromatic Lens Systems
• Composed of lens elements of varying material combined
• Neutralize dispersion, but preserving the overall refractive power
e.g
• Convex lens of high refractive power and low dispersive power
PLUS
• Concave lens of low refractive power but higher dispersive power
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44. Cont…
• The earliest achromatic lenses are made by combining elements of flint and
crown glass
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45. cont…
Filtering glasses
• Blue-blocking, red-blocking, and other coloured sunglasses
• Improve visual acuity by decreasing chromatic aberration
• They do so at the cost of reducing the colour content of the image
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46. summary
• Aberrations are deviations from stigmatic imaging.
• Its a cause of imperfections of image formation by the optical system.
• Divided into;
• Chromatic aberration
Longitudinal and transvers
• Monochromatic
Spherical aberration is a disparity in focal length for rays from a single axial
object point that are refracted at different distances from the center of the
lens
Coma is a disparity in focal length for rays from a single off-axis object point
that are refracted at different distances from the center of the lens.
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47. summary…
Astigmatism is the disparity in focal length for rays from a single object point that
are incident at different meridians of the lens
Curvature of field is a disparity in focal length for objects at different distances
from the optic axis.
Distortion is a disparity in transverse magnification for objects at different
distances from the optic axis.
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48. References
• Geometrical and visual optics 3rd edition
• Clinical optics 3rd edition; Andrew R. Elkington
• BCSC 2020-2021, section 3 optics
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49. Thank you
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