Optical Theory II ABERRATIONS Copyright 2001 -- Ellen Stoner, MALS, ABOM, NCLC.
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Aberrations
“When light from a point source goes through a correctly powered spectacle lens yet fails to create a perfect image, the cause is lens aberration.”
Brooks & Borish, Systems for Ophthalmic Dispensing, 2nd edition,
page 501
Classifications of Aberrations
• Chromatic vs. Monochromatic – Depends on the material of the lens– Requires the beam of light to contain more than
one wavelength
Classifications of Aberrations
• In Focus vs. Out of Focus – Out of focus aberrations cause fuzzy images
where clear sharp images should be– In focus aberrations cause images to be the
wrong shape (distorted).
Classifications of Aberrations
• Wide Beam vs. Narrow Beam – Wide beam aberrations are not as important
when the light goes through a narrow opening or aperture, such as the pupil of the eye.
– Narrow beam aberrations are the important aberrations when making glasses.
– Wide beam aberrations are important for optical instruments such as telescopes.
Classifications of Aberrations
• On Axis vs. Off Axis – On axis aberrations effect vision when looking
straight ahead through the lens.– Off axis aberrations effect peripheral vision.
• The lens material breaks white light into its component colors
• Why? Index of refraction varies by wavelength.
Chromatic Aberration
Longitudinal (axial)• The placement of the various focal points on the axis.
• This is the source of the Abbé value
Chromatic Aberration
• Material dependent.
• Results in out of focus image.
• wearer complains of peripheral color fringes(more pronounced off-axis).
• The higher the power of the lens, the more the chromatic aberration.
Chromatic Aberration
• Abbé value : High Abbé, low aberration
Low Abbé, high aberration
Relation between index of refraction and Abbé value is not perfect. Within a material classification it works somewhat: for example, comparing types of glass: change of index resulting from different amounts of barium in the glass.
Chromatic Aberration Abbé value index
Crown glass 58 1.523
CR-39 58 1.498
PGX 57 1.523
Spectralite 47 1.537
1.6 PGX 42 1.60
Polycarbonate 30 1.586
Brooks & Borish, Systems for Ophthalmic Dispensing 2nd ed., page 503
Chromatic Aberration• Correction:
Doublet lens (for instruments: cameras, telescopes, microscopes).
Change lens materials. AR coat. Careful placement of OC’s:
Monocular PD;OC height and pantoscopic tilt;Short vertex distance and small frame;Control edge thickness.
Consumer education.
• Peripheral rays refract more than paraxial rays.
• Correct with parabolic curves, aplanatic lens design.
• Results in out-of-focus image.• Wide beam aberration – not important in glasses
design.• On-axis aberration.
• Narrow beam aberration, therefore important in glasses lens design.
• Beam enters obliquely to lens axis, therefore effects peripheral vision.
• Creates excess power and cylinder• Also called Oblique astigmatism or Radial
astigmatism.• Correct Curve lens design for glasses corrects for
this aberration.
• Tscherning’s ellipse – a graph showing the best base curve for every Rx, to minimize marginal (oblique) astigmatism.
• Goes from about –23D to about +7D. Outside that range there is no ‘perfect’ base curve.
• Tscherning’s ellipse gives two correct base curves: one in the pl to +12 range, one in a higher plus power. We traditionally use the lower one.
Correction for Marginal (oblique) astigmatism, continued:
• Pantoscopic tilt / OC height combination.– Lower OC 1 mm for every 2 degrees
pantoscopic tilt.– Use face form in glasses where the OC’s are
decentered in.
• Aspheric design for high powers and large lenses.
• Wide beam aberration, so not important in glasses design (except very high plus Rx).
• Corrected with parabolic curves, aplanatic lens design.
• Results in out-of-focus image.• Off-axis aberration, so a peripheral vision
problem when present.• For very high plus lenses, aspheric designs
will improve coma.
Plane of focus when Marginal astigmatism is corrected
Plane of focus when Curvature of field is corrected
• Also called power error.• Light does not focus on a flat focal plane.
The focal plane is curved.• Remember the screens at drive-in movies?
They are curved, not flat, to focus the sides of the movie as well as the center.
• The retina at the back of your eye globe is not a flat plane. It is curved.
• Curvature of field is minimized with corrected curve design base curves.
• This aberration effects peripheral vision.• Petzval’s surface, or the image sphere is
the name for the curved surface when marginal (oblique) astigmatism is correct.
• Far point sphere is where the image would focus correctly.
Distortion
• Image is in focus, but not shaped the same as the object.
• Results from increased prism away from the OC of the lens.
• Solution is aspheric design lenses.
• Minor importance for glasses lenses.
Lens Aberrations
• Chromatic --------------- material dependent
• Spherical (the rest are not)
• Marginal Astigmatism
• Coma
• Curvature of Field
• Distortion ----------------- in-focus image (the rest give blurred images)
Lens Aberrations
• Chromatic
• Spherical wide beam
• Marginal Astigmatism narrow beam
• Coma wide beam
• Curvature of Field narrow beam
• Distortion
Lens Aberrations
• Chromatic
• Spherical on-axis
• Marginal Astigmatism off-axis
• Coma off-axis
• Curvature of Field on-axis
• Distortion
Lens Aberrations
• Chromatic peripheral• Spherical central• Marginal Astigmatism peripheral
(Central when pantoscopic tilt incorrect)• Coma peripheral• Curvature of Field peripheral• Distortion peripheral
Lens AberrationsIn order of importance for lens
design:
• Marginal Astigmatism
• Curvature of Field
. . . . .
• Distortion
• Chromatic aberration
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