3 - Aberrations Flashcards
Describe monochromatic aberrations
Wavelength-independent
Either:
- distort image quality (spherical, coma, radial astig)
- deform image plane (curvature of field, distortion)
Spherical aberrations
-basis for SAs
Geometric optics is based on PARAXIAL APPROXIMATIONS (near middle)
-incident rays are close to the optical axis, yielding point images for point objects
Reality = paraximal approximations are not always valid
Describe marginal rays
Rays on the periphery (not close to optical axis)
Bent MORE than central rays -> focus closer to the lens compared to central rays
BASIS OF SPHERICAL ABERRATIONS
Describe longitudinal spherial aberration
Marginal rays focus to a different location compared to paraxial rays
- essentially a point object is no longer forming a point image
- results in IMAGE BLUR
- occurs for both on- and off-axis points
- contributes to NOCTURNAL MYOPIA
Describe coma
Occurs ONLY FOR OFF-AXIS POINT SOURCES
Results from the fact that MAGNIFICATION IS VARIED as the height of incident rays above the axis is varied
-result is an asymmetric comet-shaped patch
Spherical aberration and coma
- when they’re ignored
- when they’re considered
Ignored: designing ophthalmic lenses
-bc the small pupil size only accepts paraxial rays
Considered: very high-powered lenses (+10 or more)
-necessary to compensate for spherical aberration by using aspheric lenses - modify the lens surfaces w/o changing power
General uses for aspheric lenses (4)
High-powered lenses (less than -23 or greater than +7)
Flatten lens - cosmetically appealing bc reduces magnification
Reduce weight of lens
Progressives
Describe radial astigmatism
aka oblique/marginal
Due to RAYS HITTING the lens/interface OBLIQUELY
Power is altered by this “tilt” of the lens
Tangential and sagittal rays are altered asymmetrically
Reduced by picking correct base curves
Associated with teacup and saucer image
Describe Tscherning ellipse
Collection of points on base curve vs Fv plot which shows the best value of the base curve for eliminating oblique astigmatism
Specifics vary with viewing distance and material
2 curves:
- Wollaston
- Ostwalt (flatter)
3 types of curved images produced by radial/oblique/meridonal astigmatism
Tangiential
Sagittal
Petzval
(2) can be minimized by choosing the correct base curve (primarily from the Ostwalt curve)
Curvature of field
Radial astigmatism
Describe curvature of field
aka power error
The image plane is warped even for a lens that’s not tilted -> quality of an image on a flat screen decr for larger distances
E.g. old-school projector: edges of image on screen are blurry
-due to curved image being projected
Minimizing curvature of field (2)
Good base curve (Ostwalt)
Use curved screen - e.g. retina
Describe Petzval surface
Image surface created by a system with no radial astigmatism
Still warped due to curvature of field
For a thin lens in air:
K = F ÷ n
Curvature of image surface = power of lens ÷ index of refraction
Point focal lens vs percival form lens
Point: lens corrected completely for RADIAL ASTIG (curv of field uncorrected)
Percival: lens corrected completely for CURVATURE OF FIELD (radial astig uncorrected)
Describe distortion
Does NOT cause blur or poor resolution
Results from the fact that magnification of a point object depends on the objects distance from the optical axis
Straight line objects form straight line imagees only if the line passes thru the optical axis
All other lines are curved
Problem for high powered lenses (e.g. aphakic pts)
Distortion
- minus lens
- plus lens
Minus = barrel
Plus = pincushion
Minimizing distortion
Orthoscopic doublet
Chromatic aberrations
- result from
- visible light long -> short wavelengths
- which bends more
Refractive index (n) is slightly dependent on wavelength
ROYGBIV = long to short wavelength
SHORTER wavelengths (blue, violet) bend MORE as they pass thru an interface/optical system than longer
Chromatic aberrations
-who will notice
Pts with high-powered lenses may see colored fringes around objects
Underlying concept used in red-green balance in clinical refraction
Chromatic aberration - green focuses in front of red in the eye
Abbe number
- what is it
- equation
Quantification of chromatic aberration
CA = F/υ
Chromatic aberr = power of lens ÷ abbe number
Achromatic doublet
-describe
Combo of 2 lenses: one (+), (-); each with a diff material such that chromatic aberrations cancel each other out
When considering lenses with low Abbe number, what conditions may be helpful in minimizing effects of chromatic aberration (3)
Shorter vertex distance
Monocular PDs
Sufficient panto tilt
Aberrations of most concern (3)
1) oblique/radial astig
2) curvature of field
3) distortion
