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Flashcards in 3 - Aberrations Deck (25)
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1

Describe monochromatic aberrations

Wavelength-independent

Either:
-distort image quality (spherical, coma, radial astig)
-deform image plane (curvature of field, distortion)

2

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

3

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***

4

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

5

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

6

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

7

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

8

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

9

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)

10

3 types of curved images produced by radial/oblique/meridonal astigmatism

Tangiential
Sagittal
Petzval

11

_(2)_ can be minimized by choosing the correct base curve (primarily from the Ostwalt curve)

Curvature of field
Radial astigmatism

12

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

13

Minimizing curvature of field (2)

Good base curve (Ostwalt)
Use curved screen - e.g. retina

14

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

15

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)

16

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)

17

Distortion
-minus lens
-plus lens

Minus = barrel

Plus = pincushion

18

Minimizing distortion

Orthoscopic doublet

19

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

20

Chromatic aberrations
-who will notice

Pts with high-powered lenses may see colored fringes around objects

21

Underlying concept used in red-green balance in clinical refraction

Chromatic aberration - green focuses in front of red in the eye

22

Abbe number
-what is it
-equation

Quantification of chromatic aberration

CA = F/υ

Chromatic aberr = power of lens ÷ abbe number

23

Achromatic doublet
-describe

Combo of 2 lenses: one (+), (-); each with a diff material such that chromatic aberrations cancel each other out

24

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

25

Aberrations of most concern (3)

1) oblique/radial astig
2) curvature of field
3) distortion