optical theory ii aberrations copyright 2001 -- ellen stoner, mals, abom, nclc
TRANSCRIPT
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.
Lens Aberrations
• Chromatic
• Spherical
• Marginal Astigmatism
• Coma
• Curvature of Field
• Distortion
• 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.
Lens Aberrations
• Chromatic
• Spherical
• Marginal Astigmatism
• Coma
• Curvature of Field
• Distortion
• 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.
Lens Aberrations
• Chromatic
• Spherical
• Marginal Astigmatism
• Coma
• Curvature of Field
• Distortion
• 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.
Lens Aberrations
• Chromatic
• Spherical
• Marginal Astigmatism
• Coma
• Curvature of Field
• Distortion
• 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.
Lens Aberrations
• Chromatic
• Spherical
• Marginal Astigmatism
• Coma
• Curvature of Field
• Distortion
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.
Lens Aberrations
• Chromatic
• Spherical
• Marginal Astigmatism
• Coma
• Curvature of Field
• Distortion
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